Stop depending on GCS public access for Kokoro (#2907)

We used to publish test artifacts to a Maven repo on GCS, for use by
schema tests. For this to work with Kokoro, the GCS bucket must be
accessible to all users.

To comply with the no-public-user requirement, we store the necessary
jars at at well-known bucket and map them into Kokoro. This strategy
cannot be used on the Maven repo because only a small number of files
with fixed names may be mapped. With the Maven repo, there are too many
files to map.

core/build.gradle

@@ -646,23 +646,6 @@ artifacts {
   nomulus_test testUberJar
 }
 
-publishing {
-  repositories {
-    maven {
-      url project.publish_repo
-    }
-  }
-  publications {
-    nomulusTestsPublication(MavenPublication) {
-      groupId 'google.registry'
-      artifactId 'nomulus_test'
-      version project.nomulus_version
-      artifact nomulusFossJar
-      artifact testUberJar
-    }
-  }
-}
-
 task buildToolImage(dependsOn: nomulus, type: Exec) {
   commandLine 'docker', 'build', '-t', 'nomulus-tool', '.'
 }