Remove cleanup from linux-nat.c

Remove cleanup from linux-nat.c

This removes a cleanup from linux-nat.c, replacing it with a
scope_exit.

gdb/ChangeLog:
2019-01-23  Tom Tromey  <tom@tromey.com>
	    Pedro Alves  <palves@redhat.com>

	* linux-nat.c: Include scope-exit.h.
	(cleanup_target_stop): Remove.
	(linux_nat_target::static_tracepoint_markers_by_strid): Use
	SCOPE_EXIT.