boards/arc: Remove tick rate settings for ARC hardware

The ARC timer is a MHz-scale cycle counter and works very well with
the new 10 kHz default tick rate.  Remove the settings for ARC
hardware.

Note that the nsim board definitions are left at 100 Hz.  That is a
software emulation environment that (like qemu) exposes the host clock
as "real" time and thus is subject to clock jitter due to host
scheduling.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>