This reverts commits fd5aa93a
and cf00f60b
("avcodec/kbdwin: Support arbitrary sized windows").

The change in question has only been made for libavradio.
in anticipation of merging it into the main tree. This has
not happened, so this commit reverts the changes to kbdwin
that are not used for anything else. In particular, these
functions are no longer exported (as avpriv functions);
notice that the fixed-point function has been exported
despite having never been used outside of lavc.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>