Bigtable: cleanup unused variable and add deprecation warnings for to be removed features

[AVaksman]

Fixes #7531

• some cleanup
• add deprecation warnings for backwards compatible features that will go away

[tseaver]

I think we can make this just a regular DeprecationWarning. Thanks for adding stacklevel=2!

[AVaksman]

Done, thanks

[tseaver]

@AVaksman We need to update the unit tests to catch / assert the new warnings being raised, e.g.:

    def test_foo(self):
        import warnings

        with warnings.catch_warnings(record=True) as warned:
            foo()

        self.assertEqual(len(warned), 1)
        self.assertTrue(issubclass(warned[0].category, DeprecationWarning)

[AVaksman]

@AVaksman We need to update the unit tests to catch / assert the new warnings being raised, e.g.:

    def test_foo(self):
        import warnings

        with warnings.catch_warnings(record=True) as warned:
            foo()

        self.assertEqual(len(warned), 1)
        self.assertTrue(issubclass(warned[0].category, DeprecationWarning)

Done, thanks

[tseaver]

Unrelated test failures:

• The api_core failure seems clearly unrelated to this PR: it fails in the pytype job.
• The storage failure is a known flaky system test, Storage: 'tearDownModule' flakes with 409 #7505.