style(ctb): remove errs in virtual fns (#2975)

Removes "not implemented" errors in virtual functions. By removing these
errors, developers will get compile-time errors instead of runtime
errors which are easier to catch.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/weak-pens-push.md

+---
+'@eth-optimism/contracts-bedrock': patch
+---
+
+Remove "not implemented" errors in virtual functions

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

@@ -333,9 +333,7 @@ abstract contract CrossDomainMessenger is
      *         contracts because the logic for this depends on the network where the messenger is
      *         being deployed.
      */
-    function _isSystemMessageSender() internal view virtual returns (bool) {
-        revert("CrossDomainMessenger: child contract must implement");
-    }
+    function _isSystemMessageSender() internal view virtual returns (bool);
 
     /**
      * @notice Sends a low-level message to the other messenger. Needs to be implemented by child
@@ -347,7 +345,5 @@ abstract contract CrossDomainMessenger is
         uint64 _gasLimit,
         uint256 _value,
         bytes memory _data
-    ) internal virtual {
-        revert("CrossDomainMessenger: child contract must implement");
-    }
+    ) internal virtual;
 }