test(gax): revert unnecessary test-jar inclusions

Since the `gapic-showcase` integration test now utilizes Logback's `TestAppender` natively and the custom `envVarTest` profile in the POM was removed, there is no longer a need to expose internal GAX test classes via the `test-jar`.

Author: westarle

sdk-platform-java/gax-java/gax/pom.xml

@@ -115,7 +115,6 @@
                 <include>com/google/api/gax/rpc/testing/**</include>
                 <include>com/google/api/gax/rpc/mtls/**</include>
                 <include>com/google/api/gax/util/**</include>
-                <include>com/google/api/gax/logging/**</include>
                 <include>**/native-image.properties</include>
               </includes>
             </configuration>