Use scope_exit in regcache.c

Use scope_exit in regcache.c

This removes the regcache_invalidator class in favor of a scope_exit.
This seems like an improvement (albeit a minor one) because
regcache_invalidator is only used in a single spot.

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

	* regcache.c (class regcache_invalidator): Remove.
	(regcache::raw_write): Use make_scope_exit.