transport/grpc: WithGRPCConnectionPool is a no-op for some APIs

Connection pooling is now done via transport/grpc.DialPool and is no
longer supported with transport/grpc.Dial.

See googleapis/google-cloud-go#1777 for
the list of packages that will no longer support gRPC connection
pooling after this change is merged.

Fixes #441

Change-Id: I8e05af53940d6e0af3c2acce9f81b49eaa78bd54
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/50573
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>

option/option.go

@@ -143,6 +143,7 @@ func (w withGRPCDialOption) Apply(o *internal.DialSettings) {
 
 // WithGRPCConnectionPool returns a ClientOption that creates a pool of gRPC
 // connections that requests will be balanced between.
+//
 // This is an EXPERIMENTAL API and may be changed or removed in the future.
 func WithGRPCConnectionPool(size int) ClientOption {
 	return withGRPCConnectionPool(size)

transport/grpc/dial.go

@@ -43,11 +43,10 @@ func Dial(ctx context.Context, opts ...option.ClientOption) (*grpc.ClientConn, e
 	if o.GRPCConnPool != nil {
 		return o.GRPCConnPool.Conn(), nil
 	}
-	if o.GRPCConnPoolSize != 0 {
-		// NOTE(cbro): RoundRobin and WithBalancer are deprecated and we need to remove usages of it.
-		balancer := grpc.RoundRobin(internal.NewPoolResolver(o.GRPCConnPoolSize, o))
-		o.GRPCDialOpts = append(o.GRPCDialOpts, grpc.WithBalancer(balancer))
-	}
+	// NOTE(cbro): We removed support for option.WithGRPCConnPool (GRPCConnPoolSize)
+	// on 2020-02-12 because RoundRobin and WithBalancer are deprecated and we need to remove usages of it.
+	//
+	// Connection pooling is only done via DialPool.
 	return dial(ctx, false, o)
 }