avformat/Makefile: Remove outdated AIFF demuxer dependency on iso_media

avformat/Makefile: Remove outdated AIFF demuxer dependency on iso_media

This is a result of the mov channel parsing stuff being factored out
of mov.c twice: Once in 91b782720fd0df5571775b6591bc41797d6ecf78
to isom.c and later in 3bab7cd12802dc5abf2c5cc6dec49e9e249ce204.

Also remove the isom.h header; and while just at it, remove an unused
mathematics.h inclusion.

(isom.c actually depends upon mpeg4audio from libavcodec for
avpriv_mpeg4audio_get_config2 and avpriv_mpa_freq_tab; yet there is
no configure dependency for iso_media which leads to failure of shared
builds.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>