These muxers don't depend on the WebM Chunk or the WebM DASH Manifest
muxers.

Furthermore, remove some #if checks in webm_chunk.c and webmdashenc.c.
They are always true now that webm_chunk.c and webmdashenc.c are only
compiled when their corresponding muxers are enabled.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>