Require SSE2 instead of SSE+MMX. SSE2 implies SSE+MMX is supported, so a...

...ll checks of of 'SSE' and 'MMX' can be removed.

SSE2 is supported on more than decade-old x86 CPUs. Almost(?) every x86 CPU since PS2 was released.
Certainly running the existing code on a non-SSE2 computer resulted in most of the code being disabled anyway and the emulator being incredibly slow.
This removal will hopefully allow the removal of MMX/3DNow code as it has been superceded.