chore: updated gax-java contribution doc (#1334)

Removing unnecessary files

Author: suztomo

api-common-java/CODE_OF_CONDUCT.md

@@ -13,14 +13,6 @@ APIs. It currently contains the following packages:
   formatted as Google API resource names.
 - resourcenames: Resource Name library used by generated resource name types
 
-> For new and existing Developers/ Contributors:
->
-> In December 2022, api-common-java's build tool has been migrated from gradle to maven.
-> Gradle related files are no longer being maintained and will be removed.
->
-> The artifact coordinates in Maven Central (`{{ group_id }}:{{ artifact_id }}`) remain the same.
-
-
 Java Versions
 -------------
 
@@ -31,13 +23,6 @@ The build produces Java bytecode targeted for Java 8.
 
 The project uses Maven to build.
 
-Contributing
-------------
-
-Contributions to this library are always welcome and highly encouraged.
-
-See the [CONTRIBUTING] documentation for more information on how to get started.
-
 Versioning
 ----------
 

api-common-java/CONTRIBUTING.md

@@ -24,7 +24,15 @@ use Github pull requests for this purpose.
 To build GAX:
 
 ```sh
-mvn clean install
+# Go to the repository root
+cd gapic-generator-java
+
+# Install dependency modules to local Maven repository. Note that
+# gapic-generator-java module is not a dependency of gax-java
+mvn -B -ntp install -DskipTests --projects '!gapic-generator-java,!gax-java'
+
+# Build and install gax-java
+mvn clean install -f gax-java
 ```
 
 ### The small print