Remove --with-gmp-dir and --with-mpfr-dir

The top-level configure options --with-gmp-dir and --with-mpfr-dir
were obsoleted and marked as "REMOVED" back in 2006.  I think that's
long enough ago for everyone to have updated their scripts, so this
patch removes them entirely.  While doing this, I also found one other
leftover that wasn't removed by the earlier patch.  This is also
removed here.

2022-03-28  Tom Tromey  <tromey@adacore.com>

	* configure.ac: Remove --with-mpfr-dir and --with-gmp-dir.
	* configure: Rebuild.