locales: Remove symlinks UTF8 => UTF-8

In retrospect, having an alias for UTF-8 does not bring any real benefits
and it can cause confusion. Let's remove this *.UTF8 locale symlinks
which is closer to the convention of the other BSDs.

GCC testsuite is also removing utf8 and UTF8 locales for portability
with BSD.

Submitted by: marino
Obtained from: DragonflyBSD