fix: remove grpc-testing since it's in grpc-bom (#50)

Author: stephaniewang526

pom.xml

@@ -228,11 +228,6 @@
         <version>${gax.version}</version>
         <classifier>testlib</classifier>
       </dependency>
-      <dependency>
-        <groupId>io.grpc</groupId>
-        <artifactId>grpc-testing</artifactId>
-        <version>${grpc.version}</version>
-      </dependency>
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-core</artifactId>