Removes StringCase and replaces all Locale.ENGLISH with ROOT

As we added String#toUpperCase(Locale)/toLowerCase(Locale) emulation
to GWT, we no longer need a super-sourced StringCase utility class.

I also switched all uses of Locale.ENGLISH to Locale.ROOT which is
the 'neutral' locale for locale sensitive operations.

Change-Id: I007eba55e8091564b82f81c37a1ca8c5f73c1fec