avformat/utils: Remove superfluous headers

avformat/utils: Remove superfluous headers

stdarg.h has been included in 780d7897a9c9295b43f1f0e9b49a11f99cd402c3
for ff_url_join(). This header became unnecessary when this function was
moved into a separate file in df9f22d42b0905385629a9d368bb5a1eef2b45ef.

libavutil/pixdesc.h has been included for av_get_pix_fmt_name() in
603b8bc2a109978c8499b06d2556f1433306eca7 and is unused since commit
2fb7501938b7103624c9bef740ca498258cacdab that removed the stuff belonging
to FF_API_FORMAT_PARAMETERS. Notice that this file still uses
AV_PIX_FMT_NONE and that therefore the header libavutil/pixfmt.h has
been included (this header is included in pixdesc.h as well as also in
libavutil/internal.h which is also included).

libavutil/time_internal.h has been included for gmtime_r() in commit
e7dd97b5d8cd6ea150446591f37a5946e8ab7cfb; it is unused since commit
b72a7b96f84e5f16dd93b60668aecfda99442c71 which basically moved the code
making use of gmtime_r() to libavutil/dict.c to use in
avpriv_dict_set_timestamp().

audiointerleave.h has been added in c26e58e32cf430f060209e0d6088181f4426b3ce
because of ff_interleave_compare_dts() (at that time the muxing code
was not split from utils.c yet); said function became static in commit
101e1f6ff90c3365bfde05469ae26d2ee7f71f3e, making this header redundant.

metadata.h has been mostly included for what now resides in
libavutil/dict.h. The stuff that now resides in metadata.h has only been
used briefly: From commits ed7694d8cf4633da444237f4df7efc48936419d2 to
d60a9f52eb42dc76dea9996c8ba3567ae98a9a04.

riff.h has been added in 45da8124a09d0ac5f9d8174884584c5f80309d0c
because riff.h once contained declarations for (ff_)codec_get_tag().
This was changed in bfe5454cd238b16e7977085f880205229103eccb.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>