chore(java-spanner): modernize submodule pom.xml

Author: chingor13

java-spanner/google-cloud-spanner-executor/pom.xml

@@ -202,14 +202,12 @@
       <version>2.21.0</version>
     </dependency>
 
-    <!-- Test dependencies -->
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
       <scope>test</scope>
     </dependency>
 
-    <!-- Need testing utility classes for generated gRPC clients tests -->
     <dependency>
       <groupId>com.google.api</groupId>
       <artifactId>gax</artifactId>