Remove Gradle read-only cache registration

The simple `cp -R` implementation seemed to cause corruption issues and
not all of our CI worker images have rsync installed. We'll need to sort
something else out, which might just be relying on per-BWC branch cache
directories for now.

Author: mark-vieira

.ci/packer_cache.sh

@@ -44,9 +44,3 @@ fi
 ## therefore we run main _AFTER_ we run 6.8 which uses an earlier gradle version
 export JAVA_HOME="${HOME}"/.java/${ES_BUILD_JAVA}
 ./gradlew --parallel clean -s resolveAllDependencies -Dorg.gradle.warning.mode=none -Drecurse.bwc=true
-
-## Copy all dependencies into a "read-only" location to be used by nested Gradle builds
-mkdir -p ${HOME}/gradle_ro_cache
-rsync -r ${HOME}/.gradle/caches/modules-2 ${HOME}/gradle_ro_cache
-rm ${HOME}/gradle_ro_cache/modules-2/gc.properties
-rm ${HOME}/gradle_ro_cache/modules-2/*.lock