avutil & avdevice: remove av_bprint_fd_contents()

MSVC does not allow passing file pointers between libs
This API can thus not work with MSVC and as it was very recently added
and its it was in no release its removial should not cause any problems

A better API will be implemented, but its not finished yet, this revert is
to avoid potentially blocking the release

Found-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>