Remove generator_test_wrapper for python's generator_test.

The test wrappers were another way to document nonconformant behaviour between
different python backends. We can achieve the same by removing the wrapper
script and adding an if-condition in the test itself based on
api_implementation.Type(). Since we already do that for nonconformance between
pure Python vs. C++ backends, this change makes it easier to look for UPB
nonconformance instead of going through another layer of indirection.

Temporarily, we will need to hardcode the migrated test name in test_upb.yml
because not all tests under google.protobuf.internal support UPB yet.
(UPB testing for selected tests are added in https://github.com/protocolbuffers/protobuf/commit/21e9aa6cac24870edf050eb0ff9b591380340c4d).

PiperOrigin-RevId: 712672890