chore: Update gax-grpc's testlib refrences to use Message

Author: lqiu96

gax-java/gax-grpc/src/test/java/com/google/api/gax/grpc/MockOperationsEx.java

@@ -31,7 +31,6 @@
 
 import com.google.api.core.BetaApi;
 import com.google.api.gax.grpc.testing.MockGrpcService;
-import com.google.protobuf.AbstractMessage;
 import com.google.protobuf.Message;
 import io.grpc.ServerServiceDefinition;
 import java.util.List;

gax-java/gax-grpc/src/test/java/com/google/longrunning/MockOperations.java

@@ -31,7 +31,6 @@
 
 import com.google.api.core.BetaApi;
 import com.google.api.gax.grpc.testing.MockGrpcService;
-import com.google.protobuf.AbstractMessage;
 import com.google.protobuf.Message;
 import io.grpc.ServerServiceDefinition;
 import java.util.List;

gax-java/gax-grpc/src/test/java/com/google/longrunning/MockOperationsImpl.java

@@ -31,7 +31,6 @@
 
 import com.google.api.core.BetaApi;
 import com.google.longrunning.OperationsGrpc.OperationsImplBase;
-import com.google.protobuf.AbstractMessage;
 import com.google.protobuf.Empty;
 import com.google.protobuf.Message;
 import io.grpc.stub.StreamObserver;

gax-java/gax-grpc/src/test/java/com/google/longrunning/OperationsClientTest.java

@@ -39,7 +39,6 @@
 import com.google.api.gax.rpc.InvalidArgumentException;
 import com.google.common.collect.Lists;
 import com.google.longrunning.OperationsClient.ListOperationsPagedResponse;
-import com.google.protobuf.AbstractMessage;
 import com.google.protobuf.Duration;
 import com.google.protobuf.Empty;
 import com.google.protobuf.Message;