Remove now-unused _dbus_validate_signature()

Remove now-unused _dbus_validate_signature()

All callers should use _dbus_validate_signature_with_reason() directly.
The only remaining callers were this function's own tests.

As a side benefit, this commit removes a TODO pointing out that this
function did not follow normal DBusString conventions, by considering
a length outside the bounds of the DBusString to be an ordinary
lack of validity rather than a fatal programming error.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101568