fix(ctb): remove DefaultValues (#2961)

Unnecessary library, only being used in a single place now and it can
just be an internal constant. I see no strong reason for this library to
continue to exist.

.changeset/friendly-donuts-kiss.md

+---
+'@eth-optimism/contracts-bedrock': patch
+---
+
+Remove unnecessary DefaultValues library

packages/contracts-bedrock/contracts/libraries/DefaultValues.sol

-// SPDX-License-Identifier: MIT
-pragma solidity ^0.8.9;
-
-/**
- * @title DefaultValues
- */
-library DefaultValues {
-    // The default x-domain message sender being set to a non-zero value makes
-    // deployment a bit more expensive, but in exchange the refund on every call to
-    // `relayMessage` by the L1 and L2 messengers will be higher.
-    address internal constant DEFAULT_XDOMAIN_SENDER = 0x000000000000000000000000000000000000dEaD;
-}

packages/contracts-bedrock/contracts/test/L1CrossDomainMessenger.t.sol

@@ -7,7 +7,6 @@ import { L2OutputOracle_Initializer } from "./L2OutputOracle.t.sol";
 
 /* Libraries */
 import { AddressAliasHelper } from "../vendor/AddressAliasHelper.sol";
-import { DefaultValues } from "../libraries/DefaultValues.sol";
 import { PredeployAddresses } from "../libraries/PredeployAddresses.sol";
 import { CrossDomainUtils } from "../libraries/CrossDomainUtils.sol";
 import { WithdrawalVerifier } from "../libraries/WithdrawalVerifier.sol";

packages/contracts-bedrock/contracts/test/L2CrossDomainMessenger.t.sol

@@ -10,7 +10,6 @@ import { L2OutputOracle } from "../L1/L2OutputOracle.sol";
 import { L2CrossDomainMessenger } from "../L2/L2CrossDomainMessenger.sol";
 import { L1CrossDomainMessenger } from "../L1/L1CrossDomainMessenger.sol";
 import { CrossDomainHashing } from "../libraries/CrossDomainHashing.sol";
-import { DefaultValues } from "../libraries/DefaultValues.sol";
 
 contract L2CrossDomainMessenger_Test is Messenger_Initializer {
     // Receiver address for testing

packages/contracts-bedrock/contracts/universal/CrossDomainMessenger.sol

@@ -11,7 +11,6 @@ import {
     ReentrancyGuardUpgradeable
 } from "@openzeppelin/contracts-upgradeable/security/ReentrancyGuardUpgradeable.sol";
 import { ExcessivelySafeCall } from "excessively-safe-call/src/ExcessivelySafeCall.sol";
-import { DefaultValues } from "../libraries/DefaultValues.sol";
 import { CrossDomainHashing } from "../libraries/CrossDomainHashing.sol";
 
 /**
@@ -83,6 +82,13 @@ abstract contract CrossDomainMessenger is
      */
     uint256 internal constant RELAY_GAS_BUFFER = RELAY_GAS_REQUIRED - 5000;
 
+    /**
+     * @notice Initial value for the xDomainMsgSender variable. We set this to a non-zero value
+     *         because performing an SSTORE on a non-zero value is significantly cheaper than on a
+     *         zero value.
+     */
+    address internal constant DEFAULT_XDOMAIN_SENDER = 0x000000000000000000000000000000000000dEaD;
+
     /**
      * @notice Mapping of message hashes to boolean receipt values. Note that a message will only
      *         be present in this mapping if it failed to be relayed on this chain at least once.
@@ -152,10 +158,7 @@ abstract contract CrossDomainMessenger is
      * @return Address of the sender of the currently executing message on the other chain.
      */
     function xDomainMessageSender() external view returns (address) {
-        require(
-            xDomainMsgSender != DefaultValues.DEFAULT_XDOMAIN_SENDER,
-            "xDomainMessageSender is not set"
-        );
+        require(xDomainMsgSender != DEFAULT_XDOMAIN_SENDER, "xDomainMessageSender is not set");
 
         return xDomainMsgSender;
     }
@@ -287,7 +290,7 @@ abstract contract CrossDomainMessenger is
             0,
             _message
         );
-        xDomainMsgSender = DefaultValues.DEFAULT_XDOMAIN_SENDER;
+        xDomainMsgSender = DEFAULT_XDOMAIN_SENDER;
 
         if (success == true) {
             successfulMessages[versionedHash] = true;
@@ -312,7 +315,7 @@ abstract contract CrossDomainMessenger is
     function _initialize(address _otherMessenger, address[] memory _blockedSystemAddresses)
         internal
     {
-        xDomainMsgSender = DefaultValues.DEFAULT_XDOMAIN_SENDER;
+        xDomainMsgSender = DEFAULT_XDOMAIN_SENDER;
         otherMessenger = _otherMessenger;
 
         for (uint256 i = 0; i < _blockedSystemAddresses.length; i++) {