Remove resume_section_map_updates_cleanup

This removes resume_section_map_updates_cleanup, replacing it with a
scoped_restore.

Tested by the buildbot.

gdb/ChangeLog
2018-06-18  Tom Tromey  <tom@tromey.com>

	* objfiles.h (inhibit_section_map_updates): Update.
	(resume_section_map_updates, resume_section_map_updates_cleanup):
	Remove.
	* solib-svr4.c (svr4_handle_solib_event): Update.
	* objfiles.c (inhibit_section_map_updates): Return
	scoped_restore_tmpl<int>.
	(resume_section_map_updates, resume_section_map_updates_cleanup):
	Remove.