Remove Resource._Implementations.loadXXX

As mentioned in #7140 `Resource._Implementations.loadAndExecuteScript` could be simplified to not take a `deferred`. It also currently takes a `functionName` parameter that isn't used.

While this is an easy change to make, the problem is that the tests really abuse `Resource._Implementations.loadXXX` spies for mocking and cleaning up the code means the tests all have to be updated, which may not be too much work but requires fully understanding what's going on in them.

Start by locating Resource._Implementations.loadAndExecuteScript and the tests that spy on Resource._Implementations.loadXXX. Read the existing mocking and cleanup paths before removing the unused deferred and functionName parameters. Done means the implementation and all affected tests no longer depend on those parameters and the relevant tests pass.

Written by the indexing model from the issue text.