drivers: timer: cortex_m_systick remove dead code

Removed CONFIG_INT_LATENCY_BENCHMARK dead code.
CONFIG_INT_LATENCY_BENCHMARK depends on ARCH="x86", it will never
be true for ARM code.

Change-Id: Ia5779a69b1bf670ebb140c2923c9fe0af6b781d4
Signed-off-by: Javier B Perez <javier.b.perez.hernandez@intel.com>