contracts-bedrock: delete unused import (#2929)

In `OptimismMintableERC20.t.sol`, the RLP lib is imported
and unused. This deletes the import.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

packages/contracts-bedrock/contracts/test/OptimismMintableERC20.t.sol

@@ -2,7 +2,6 @@
 pragma solidity 0.8.10;
 
 import { Bridge_Initializer } from "./CommonTest.t.sol";
-import { LibRLP } from "./Lib_RLP.t.sol";
 import "../universal/SupportedInterfaces.sol";
 import { IERC165 } from "@openzeppelin/contracts/utils/introspection/IERC165.sol";