[docs] Removing deprecated gRPC codegen from the documentation

Summary:
This diff removes the deprecated gRPC codegen from the documentation.

Linear task: ENG-2394

Test Plan: No tests, just docs.

Reviewers: jon, tomek, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5942