Remove the old code using MMX registers.

Remove the old code using MMX registers.

There are only 8 MMX registers, each 64-bit wide, and they alias the
x87 registers. The access to the MMX register cannot use the new
VEX-prefix instructions either.

All of the functions being replaced are either present in the
qdrawhelper_sse2.cpp and qdrawhelper_ssse3.cpp files, or the plain C++
function in qdrawhelper.cpp is vectorised when compiled with
-ftree-vectorize (enabled in -O3), if SSE2 support is enabled.

All x86-64 processors have SSE2, so this is a net improvement for
64-bit builds. For 32-bit builds, without further support this will
cause the code to use non-vector or x87 instructions, which aren't the
best. The solution will come in another commit.

Change-Id: I4a22d8a2516b79172867510202d0fd627db54807
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>