h264: eliminate ff_h264_set_parameter_from_sps()

That function currently does two things -- reinitializing the DSP
contexts and setting low_delay based on the SPS values.

The former more appropriately belongs in h264_slice_header_init(), while
the latter only really makes sense in decode_slice_header().

The third call to ff_h264_set_parameter_from_sps(), done immediately
after parsing a new SPS, appears to serve no useful purpose, so it is
just dropped.

Also, drop now unneeded H264Context.cur_chroma_format_idc.