[google_sign_in] Clean up Java code (flutter#8241)

Removes the legacy public native interface to the sign-in plugin, which was inherently linked to raw method channels (which we no longer use) and the structure of the Google Sign-In native SDK (which is deprecated), so we don't want to continue supporting.

I (unsurprisingly, as native->native plugin dependencies are very rare) can't find any evidence that any public plugin has ever directly used this Java code. The original use case this was added for was internal, and no longer exists (see b/158361263).

The test file being deleted is specific to the legacy interface. There is already an almost-exact duplicate of that test file that tests the Pigeon interface.

Author: stuartmorgan-g

packages/google_sign_in/google_sign_in_android/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 6.1.34
+
+* Removes unnecessary native code.
+
 ## 6.1.33
 
 * Updates Pigeon for non-nullable collection type support.