Revert "avformat/flvenc: Allow muxing video codecs which are not explicitly supported by the muxer"

This commit has no known use case ATM as there are no unsupported video codecs in flv and could
theoretically be use to generate broken files allthough that would be not entirely easy as
tags/codecs still get sanity checked

This reverts commit 76f4b117807c30a528539fc5c7a7e35cb288cfcf.
