Remove dead abstract contract Impl from DummyImplementation mock (#6073)

Author: sashass1315

contracts/mocks/DummyImplementation.sol

@@ -5,10 +5,6 @@ pragma solidity ^0.8.21;
 import {ERC1967Utils} from "../proxy/ERC1967/ERC1967Utils.sol";
 import {StorageSlot} from "../utils/StorageSlot.sol";
 
-abstract contract Impl {
-    function version() public pure virtual returns (string memory);
-}
-
 contract DummyImplementation {
     uint256 public value;
     string public text;