Revert "ssh-agent: Fix leak in search_keys_like_attributes()"

This reverts commit d177a8b5.

This breaks the ssh agent as this causes it to fail to remember unlocked
keys, and to always ask for a key passphrase when attempting an ssh
connection.
From https://bugzilla.gnome.org/show_bug.cgi?id=744280 :
« ssh foo.example.com is always asking to unlock the corresponding key
while before this commit this works fine.

gkd-ssh-agent-ops.c:op_sign_request has:
result = unlock_and_sign (session, key, mech, hash, n_hash, &n_result,
&error);
g_object_unref (session);
g_object_unref (key);

After the leak fix mentioned above, unreffing 'key' causes the last
reference to be released, so it's destroyed, and this locks the just
unlocked key.
Removing the g_object_unref() avoids this issue, but readds a leak. I
assume 'key' should be added somewhere so that it's kept alive, but I'm
not really sure where/how »