lavc: remove old unused audio conversion functions.
Merge commit 'c9a13a289d0e1607387854127476813a1ee3d34b'

* commit 'c9a13a289d0e1607387854127476813a1ee3d34b':
  lavc: remove old unused audio conversion functions.

Conflicts:
	libavcodec/audioconvert.c
	libavcodec/audioconvert.h

Merge only for metadata.
keeping the old API longer reduces the need for applications to be continuously
updated.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/cook: use av_freep()

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avcodec/audioconvert: deprecate API

Note, the header was not installed so useage of this should be limited
but its not 0, there are some random pieces of code using it according
to google.

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