Clean up some QT_HAVE_xxx macros in qcompilerdetection.h

Clean up some QT_HAVE_xxx macros in qcompilerdetection.h

We no longer have code doing MMX or 3dNow! on Qt 5, so nothing defines
those macros. If nothing defines them, we don't need to undef
them. Note that the SSE case is a mislabel: the old "SSE" code on Qt 4
wasn't real SSE, it was MMX with some instructions added at the same
time as SSE was added. It's MMX that Windows 64 doesn't support.

As for QT_HAVE_ARMV6, this macro is replaced by the sub-arch detection
in qprocessordetection.h.

Change-Id: Ic3b00e1533e6b4cea32ba7824233de0a5c0fb32b
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>