Remove make_cleanup_restore_current_thread from gdbserver

Remove make_cleanup_restore_current_thread from gdbserver

This removes make_cleanup_restore_current_thread from gdbserver,
replacing it with a use of scoped_restore.

2018-02-08  Tom Tromey  <tom@tromey.com>

	* linux-low.c (install_software_single_step_breakpoints): Use
	make_scoped_restore.
	* inferiors.c (make_cleanup_restore_current_thread): Remove.
	(do_restore_current_thread_cleanup): Remove.
	* gdbthread.h (make_cleanup_restore_current_thread): Don't
	declare.