contrib/credential: remove 'gnome-keyring' credential helper

contrib/credential: remove 'gnome-keyring' credential helper

libgnome-keyring was deprecated in 2014 (in favor of libsecret), more
than nine years ago [1].

The credential helper implemented using libgnome-keyring has had a small
handful of commits since 2013, none of which implemented or changed any
functionality. The last commit to do substantial work in this area was
15f7221686 (contrib/git-credential-gnome-keyring.c: support really
ancient gnome-keyring, 2013-09-23), just shy of nine years ago.

This credential helper suffers from the same `fgets()`-related injection
attack (using the new "wwwauth[]" feature) as in the previous commit.
Instead of patching it, let's remove this helper as deprecated.

[1]: https://mail.gnome.org/archives/commits-list/2014-January/msg01585.html

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>