Delete deprecated `sapi::Sandbox::GetRpcChannel` and its remaining call sites.
Delete deprecated `sapi::Sandbox::GetRpcChannel` and its remaining call sites.

PiperOrigin-RevId: 389968873
Change-Id: Ia72e0064fa57679180f9c406f96266473f8461c2

google/sandboxed-api,google/sandboxed-api,google/sandboxed-api,google/sandboxed-api