lavc/ccaption_dec: remove usage of avctx->time_base

lavc/utils already rescales avpkt->pts to sub->pts in AV_TIME_BASE_Q
before calling the decode callback. This prevents from rescaling again
into the decoder, and avoid the use of avctx->time_base which will
disappear in the incoming codecpar merge.

This commit also replaces the use of "20 centisecond" (ass time base)
with "200 ms".
