Revert "Deprecated GrpcClientBuilder#MultiClientBuilder" (#2128)

This reverts commit 380c2ad.

`GrpcClientBuilder#MultiClientBuilder` is a clean way to reuse the underlying transport between different service clients and at the moment there is no practical alternative to replace it without the risk of losing the default filters.

servicetalk-grpc-api/src/main/java/io/servicetalk/grpc/api/GrpcClientBuilder.java

@@ -491,12 +491,9 @@ public final GrpcClientBuilder<U, R> appendHttpClientFilter(Predicate<StreamingH
      * instance.
      *
      * @return A blocking <a href="https://www.grpc.io">gRPC</a> client.
-     * @deprecated Use {@link GrpcClientFactory#newClient(GrpcClientCallFactory)}
-     * or {@link GrpcClientFactory#newBlockingClient(GrpcClientCallFactory)}
-     * and provide a custom {@link GrpcClientCallFactory} instead.
      */
-    @Deprecated
     public final MultiClientBuilder buildMulti() {
+
         GrpcClientCallFactory callFactory = newGrpcClientCallFactory();
         return new MultiClientBuilder() {
             @Override
@@ -583,11 +580,7 @@ protected void doAppendHttpClientFilter(Predicate<StreamingHttpRequest> predicat
     /**
      * An interface to create multiple <a href="https://www.grpc.io">gRPC</a> clients sharing the
      * same underlying transport instance.
-     * @deprecated Use {@link GrpcClientFactory#newClient(GrpcClientCallFactory)}
-     * or {@link GrpcClientFactory#newBlockingClient(GrpcClientCallFactory)}
-     * and provide a custom {@link GrpcClientCallFactory} instead.
      */
-    @Deprecated
     public interface MultiClientBuilder {
 
         /**