remove duplicated parser, people who pass random gibblets of the bitstream...

remove duplicated parser, people who pass random gibblets of the bitstream into the decoder will have to pass it through a AVParser like for all other codecs too
remove silly internal buffering architecture (removes 1 memcpy() of the bitstream)
mp3on4 and mp3adu untested

Originally committed as revision 6098 to svn://svn.ffmpeg.org/ffmpeg/trunk