chore: remove duplicate declaration for com.google.api:gax-grpc:jar:testlib (#11546)

duplicate declaration for gax-grpc. Exact same dependency at
[L126-131](https://github.com/googleapis/google-cloud-java/blob/a3eafb2a66bf8a082d91bf4550b5aab7403f8084/java-notebooks/google-cloud-notebooks/pom.xml#L126-L131)

Author: zhumin8

java-notebooks/google-cloud-notebooks/pom.xml

@@ -110,13 +110,6 @@
       <artifactId>proto-google-iam-v1</artifactId>
     </dependency>
     <!-- Need testing utility classes for generated gRPC clients tests -->
-    <dependency>
-      <groupId>com.google.api</groupId>
-      <artifactId>gax-grpc</artifactId>
-      <classifier>testlib</classifier>
-      <scope>test</scope>
-    </dependency>
-    <!-- Need testing utility classes for generated gRPC clients tests -->
     <dependency>
       <groupId>com.google.api</groupId>
       <artifactId>gax</artifactId>