lavf: Remove codec_tag from dashenc and smoothstreamingenc

Skip the codec_tag altogether here, to let the user (try to) set
whichever codec/tag is preferred; the individual chained muxer will
reject invalid codecs anyway.

(cherry picked from commit 61f589e31e84ae02d7ac6837f30f19c437b1fc2e)
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
