avcodec/mpegaudio_parser: Discard ID3v1 tag at the end

Ideally this should be discarded by the demuxer but this is not
possible without fully parsing which would be then very similar
to this. The current ID3v1 discard code in the demuxer does not work
and will be removed in a subsequent commit

The discard code could be adjusted if needed to also discard tags at
other locations than the end or to limit this possibly to input
from the mp3 demuxer or even to move the discarding to the
decoder.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>