ARM: drop <mach/timex.h> for !ARCH_MULTIPLATFORM, too

While <mach/timex.h> isn't used for multi-platform builds since long it
still is for "normal" builds. As the previous patches fix all sites to
not make use of this per-platform file, it can go now for good also for
platforms that are not (yet) converted to multi-platform.

While at it there are no users of CLOCK_TICK_RATE any more, so also drop
the dummy #define.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>