Remove arm_override_mode

GDB can determine the kind of single step breakpoint by gdbarch
breakpoint_kind_from_current_state, so global variable
arm_override_mode is no longer needed.  This patch removes it.

gdb:

2016-11-03  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (arm_override_mode): Remove.
	(arm_pc_is_thumb): Update.
	(arm_insert_single_step_breakpoint): Update.