remove '-Xfatal-warnings' from gradle settings

[rabbah]

this prevents the use of @deprecated annotations upstream to make it possible to transition interfaces or make changes.

This likely affects other repositories as well.

@markusthoemmes opinion?

[markusthoemmes]

👍

Unfortunately, Scala doesn't seem to have finer grained support, i.e. to disable just the warnings for deprecation, neither does it have a @SuppressWarnings annotation.