classes/mirrors: remove bogus gnutls mirror

classes/mirrors: remove bogus gnutls mirror

This mirror entry which maps to itself plus a slash, if matched, put the
fetcher into a circular loop until the stack space is exhausted. A patch
has been sent to fix this issue in BitBake, but we should remove the
bogus entry as well.

(Note that this entry does not actually trigger the issue with current
master because the gnutls recipe now uses GNU_MIRROR instead of
ftp.gnutls.org, thus the bogus mirror entry is not matched.)

(From OE-Core rev: 0de1827a9601143b090f751ea702fdb65a936b77)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>