Remove unused mock contracts from SafeToL2Migration tests (#837)

This pull request focuses on cleaning up the `SafeToL2Migration.spec.ts`
test file by removing unused imports and redundant mock setup code.
These changes streamline the test setup and improve code readability.

Code cleanup and simplification:

* Removed the unused `getMock` import from
`test/libraries/SafeToL2Migration.spec.ts`.
* Deleted the setup and usage of `validGuardMock` and `invalidGuardMock`
in the `SafeToL2Migration` library tests, as they were not used in the
actual tests.
[[1]](diffhunk://#diff-75100fb7649ea61385b33880528c60fc7ed991face5d9285fa01ebaeb4616221L79-L86)
[[2]](diffhunk://#diff-75100fb7649ea61385b33880528c60fc7ed991face5d9285fa01ebaeb4616221L116-L117)

Author: mmv08

test/libraries/SafeToL2Migration.spec.ts

@@ -1,7 +1,7 @@
 import { expect } from "chai";
 import hre, { ethers, deployments } from "hardhat";
 import { AddressZero } from "@ethersproject/constants";
-import { getSafe, getSafeSingletonAt, getMock, getAbi, getSafeL2Singleton, getCompatFallbackHandler } from "../utils/setup";
+import { getSafe, getSafeSingletonAt, getAbi, getSafeL2Singleton, getCompatFallbackHandler } from "../utils/setup";
 import deploymentData from "../json/safeDeployment.json";
 import { Safe, SafeL2 } from "../../typechain-types";
 import {
@@ -76,14 +76,6 @@ describe("SafeToL2Migration library", () => {
         const singleton130 = await getSafeSingletonAt(SAFE_SINGLETON_130_ADDRESS);
         const singleton141 = await getSafeSingletonAt(SAFE_SINGLETON_141_ADDRESS);
 
-        const guardContract = await hre.ethers.getContractAt("ITransactionGuard", AddressZero);
-        const guardEip165Calldata = guardContract.interface.encodeFunctionData("supportsInterface", ["0xe6d7a83a"]);
-        const validGuardMock = await getMock();
-        await validGuardMock.givenCalldataReturnBool(guardEip165Calldata, true);
-
-        const invalidGuardMock = await getMock();
-        await invalidGuardMock.givenCalldataReturnBool(guardEip165Calldata, false);
-
         const safeWith1967Proxy = await getSafeSingletonAt(
             await hre.ethers
                 .getContractFactory("UpgradeableProxy")
@@ -113,8 +105,6 @@ describe("SafeToL2Migration library", () => {
             safeWith1967Proxy,
             migration,
             signers,
-            validGuardMock,
-            invalidGuardMock,
         };
     });