[context] Move Google context to arena based contexts (#36777)

Closes #36777

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36777 from ctiller:ctx3 197c74753b
PiperOrigin-RevId: 639049684

src/core/lib/channel/context.h

@@ -59,9 +59,6 @@ typedef enum {
   /// the server.
   GRPC_CONTEXT_BACKEND_METRIC_PROVIDER,
 
-  /// Special Google context
-  GRPC_CONTEXT_GOOGLE,
-
   GRPC_CONTEXT_COUNT
 } grpc_context_index;