Remove useless x86 fallbacks.

The NDK ABI requires that you support SSE2, and the build system won't let you
build with ARCH_X86_HAVE_SSE2 set to false. So let's stop pretending this
constant is actually a variable, and let's remove the corresponding dead code.

Also, the USE_SSE2 and USE_SSE3 macros are unused, so let's not bother
setting them.

Change-Id: I40b501d998530d22518ce1c4d14575513a8125bb