avcodec/x86/huffyuvencdsp: Remove obsolete MMX function

The only systems which benefit from ff_diff_int16_mmx are truely
ancient 32bit x86s as all other systems use at least the SSE2 versions
(this includes all x64 cpus (which is why this code is restricted
to x86-32)).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>