x64 always has MMX, MMXEXT, SSE and SSE2 and this means
that some functions for MMX, MMXEXT and 3dnow are always
overridden by other functions (unless one e.g. explicitly
disables SSE2) for x64. So given that the only systems that
benefit from ff_ssd_int8_vs_int16_mmx are truely ancient
32bit x86s it is removed.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>