Remove Resource._Implementations.loadAndExecuteScript

[mramato]

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.loadAndExecuteScript 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.