[Cast Embedder] Remove gRPC Multizone from CWR

This CL removes the current multizone-related code used in the CWR, as
it does not work with the current infrastructure and will cause issues
in the migration from `//chromecast` to `//components`

Bug: b/244455581
Change-Id: I3e74a129b855a4a8fc310208d0f80005e220e735
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3867380
Reviewed-by: Vigen Issahhanjan <vigeni@google.com>
Commit-Queue: Ryan Keane <rwkeane@google.com>
Cr-Commit-Position: refs/heads/main@{#1041771}