Log Message:
Remove obsolete lang/gnat-aux

The gnat-aux package is based on gcc-4.6, which has been unsupported
upstream since the last point release on 25 March 2011.

Nothing depends on it. The lang/gcc-aux package (based on gcc-4.9) took
over its tasks with the "ada" support a few years ago.  A new package,
lang/gcc5-aux is coming soon.

There's no technical reason to use lang/gnat-aux over lang/gcc-aux or
lang/gcc5-aux (hopefully nobody has been!) so just remove it outright
with the recommendation to switch to a newer Ada compiler package.