arch: arm: minor clean-up in irq_init.c and timing_info_bench.c

- Remove redundant inclusions in irq_init.c
- Remove comment about thread_abort function,
  which does not belong in this file (probably
  left-out during code refactoring)
- Include arm cmsis.h only under #ifdef CONFIG_ARM

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>