mad: remove id3tag dependency used by in practice unreachable legacy code

mad: remove id3tag dependency used by in practice unreachable legacy code

The mad mp3 decoder element shouldn't parse tags at all really, but we
have so far kept this code around for backwards-compatibility reasons
for people building manual pipelines for some reason. However, as it
turns out that code has never actually worked in 0.10 in practice,
since it only gets executed if mad_frame_decode() returns LOSTSYNC,
which doesn't actually seem to happen any more though because of the
preceding mad_header_decode(), which will discover and report the
sync loss if it runs into a tag and make mad_frame_decode() try to
resync right away.

Discovered this while trying to make it use gst_tag_list_from_id3v2_tag().