Remove need_step_over from struct lwp_info

Hi,
I happen to see that field need_step_over in struct lwp_info is only
used to print a debug info.  need_step_over is set in linux_wait_1
when breakpoint_here is true, however, we check breakpoint_here too in
need_step_over_p and do the step over.  I think we don't need field
need_step_over, and check breakpoint_here directly in need_step_over_p.

This field was added in this patch
https://sourceware.org/ml/gdb-patches/2010-03/msg00605.html and the code
wasn't changed much since then.

This patch is to remove it.

gdb/gdbserver:

2016-04-28  Yao Qi  <yao.qi@linaro.org>

	* linux-low.h (struct lwp_info) <need_step_over>: Remove.
	* linux-low.c (linux_wait_1): Update.
	(need_step_over_p): Likewise.