JCLOUDS-736: Revert "Fail build on checkstyle warnings"

This reverts commit 9384575d1ee4c2e68ff503b237dfedf8ab6b14bb.  This
caused errors of the form:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-checkstyle-plugin:2.12.1:check (checkstyle) on project jclouds-core: Failed during checkstyle execution: Unable to process configuration file at location: resources/checkstyle.xml: Cannot create file-based resource:invalid distance too far back -> [Help 1]

Likely due to running checkstyle during the validate phase which runs
before the package phase where checkstyle would find its resource jar.