_dbus_message_set_signature: Delete unused function

_dbus_message_set_signature: Delete unused function

If this is reinstated it will need some checks. In particular, it
was using _dbus_check_is_valid_signature() in an unsafe way:
_dbus_check_is_valid_signature() cannot be used in a
_dbus_return_val_if_fail() check because it does not distinguish
between error by the caller, and out-of-memory conditions.

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