Hooray C++17!

This is obsolete since C++17.

This template was never instantiated, so the problem never occurred.

Some FFmpeg filters change the sample format, and since MPD assumes
this never happens, this results in loud noise instead of music.  This
commit finally implements the TODO comment by sending one frame of
silence to the filter and checking the output frame's format.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1009