videomixer: remove dead code

videomixer: remove dead code

While it seems to keep a compile time selection, I traced it
to some code copied from videoconvert, where it was removed,
with the following comment:

    Also remove the high-quality I420 to BGRA fast-path as it needs
    the same fix, which causes an additional instruction, which causes
    orc to emit more than 96 variables, which then just crashes.
    This can only be fixed in orc by breaking ABI and allowing more
    variables.

Thus, I remove it here as well.

Coverity 206064