Cortex-M: Delete an unnecessary local variable

Cortex-M: Delete an unnecessary local variable

The dhcsr_save variable was used to save the value of
cortex_m->dcb_dhcsr so it could be restored later. However, all writes
in between the save and the restore use mem_ap_write_atomic_u32, not
cortex_m_write_debug_halt_mask, which means cortex_m->dcb_dhcsr isn’t
changed anyway. Delete the unnecessary local.

Change-Id: I064a3134e21398e1ecfc9f1fa7efd7b020b52341
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4240
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>