decoder/ffmpeg: remove "rtsp://" from the list of supported protocols

FFmpeg implements RTSP as a demuxer, not as a protocol handler.  Thus,
avio_open() cannot be used, and our input plugin cannot handle RTSP.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/930

src/input/plugins/FfmpegInputPlugin.cxx

@@ -140,7 +140,6 @@ FfmpegInputStream::Seek(offset_type new_offset)
 static constexpr const char *ffmpeg_prefixes[] = {
 	"gopher://",
 	"rtp://",
-	"rtsp://",
 	"rtmp://",
 	"rtmpt://",
 	"rtmps://",