contracts-bedrock: remove storage spacers (#2907)

Remove storage spacers in the xdomainmessengers.
These are no longer necessary because the storage
layout has changed significantly and we will need
to handle that during state surgery anyways.
Inheriting from upgradable contracts with spacers
caused the storage layout to diverge from the existing
contracts.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/old-seas-jump.md

+---
+'@eth-optimism/contracts-bedrock': patch
+---
+
+Remove storage slot buffer in xdomain messengers

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

@@ -66,12 +66,6 @@ abstract contract CrossDomainMessenger is
      * Variables *
      *************/
 
-    // blockedMessages in old L1CrossDomainMessenger
-    bytes32 internal REMOVED_VARIABLE_SPACER_1;
-
-    // relayedMessages in old L1CrossDomainMessenger
-    bytes32 internal REMOVED_VARIABLE_SPACER_2;
-
     /// @notice Mapping of message hash to boolean success value.
     mapping(bytes32 => bool) public successfulMessages;