Remove stop_registers

Remove stop_registers

Now stop_registers are no longer used and it can be removed.

I am not much sure what 'proceed_to_finish' really means now so I make a wild
guess while updating comments about it.

gdb/ChangeLog
2015-05-13  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdbthread.h (struct thread_control_state): Update comment for
	proceed_to_finish.
	* infcall.c (run_inferior_call): Update comment about
	proceed_to_finish.
	* infcmd.c (get_return_value): Update comment about stop_registers.
	(finish_forward): Update comment about proceed_to_finish.
	* infrun.c (stop_registers): Remove.
	(clear_proceed_status, normal_stop): Remove stop_registers handling.
	* infrun.h (stop_registers): Remove.