Look into possible hangs in Python's unit._rpc_test.RPCTest at tearDown w/o explicit server stop

[soltanmm-google]

What it says on the tin. Remove the tearDown introduced by #8628, figure out what might or might not be flakily causing the tests to hang for an apparently finite (but long) time.

[nathanielmanistaatgoogle]

Looking further: we should resume explicitly testing server-shutdown-on-garbage-collection-in-Python-interpreters-with-deterministic-garbage-collection with the thought that even though we'd like applications to never explicitly depend on the behavior we would want to have confidence that if they accidentally forgot to stop their server they'd likely have a better debugging experience.

[soltanmm-google]

(specifically for CPython, where we think we understand the garbage collector's behavior)

[soltanmm-google]

Hangs still occur even with explicit stop, but oddly only with certain combinations of tests under unit._rpc_test running.

EDIT: I still haven't figured out what those combinations exactly are.

[soltanmm-google]

Seen: https://grpc-testing.appspot.com/job/gRPC_master_macos/869/

[nathanielmanistaatgoogle]

Seen again in gRPC_master_macos build 1111.

[nathanielmanistaatgoogle]

Punting from 1.1; this fire just isn't burning hot enough.

Also removing the BUILDPONY label; this isn't actually about currently failing tests but rather is about investigating in the future making a potential modification to our tests and then observing whether and how they then begin to fail.