Revert addition of un-suffixed pip CLI entry points for invocation check on Windows

[pradyunsg]

Closes #7358
Reopens #6841
Partially reverts #6864

[pradyunsg]

Note: @uranusjr and I investigated why the tests failed and it seems like calls made from script.run are resulting in sys.argv[0] to not include .exe suffix. The tests were working earlier, since the .exe suffix wasn't required to pass; i.e. it used the non-suffixed name to pass even when checking a different suffix. :)

I've dropped all the tests since they're not valid tests that are not useful as-is without significant investigation / reworking or are tests for functionality that doesn't exist.