Remove test code related to `PUBSUB_EMULATOR_HOST` from `client.py`

[ohmayr]

Currently, the emulator code (for testing purposes) is set in client.py i.e. if PUBSUB_EMULATOR_HOST is set:

• api endpoint is set to the PUBSUB_EMULATOR_HOST.
• credentials are set to AnonymousCredentials.
• channel is set to insecure_channel.

Remove this code from the client files and include it in the test files directly. The exact code location can be looked up by searching for this issue.

[mukund-ananthu]

Checking with ohmayr internally.

[mukund-ananthu]

Confirmed this is a clean up task to declutter the library code. Downgrading to P3