lavf: Remove codec_tag from dashenc and smoothstreamingenc

Currently, the tags enforced and set on the segmenter muxer level
mismatch what the mp4/ismv muxer uses (since 713efb2c0d013).

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.

Signed-off-by: Martin Storsjö <martin@martin.st>
