Remove directive-searched cleanups

Remove directive-searched cleanups

This removes a few cleanups related to the "searched" field in
struct using_direct, replacing these with scoped_restore.

gdb/ChangeLog
2017-11-04  Tom Tromey  <tom@tromey.com>

	* cp-namespace.c (reset_directive_searched): Remove.
	(cp_lookup_symbol_via_imports): Use scoped_restore.
	* cp-support.c (reset_directive_searched): Remove.
	(make_symbol_overload_list_using): Use scoped_restore.
	* d-namespace.c (d_lookup_symbol_imports): Use scoped_restore.
	(reset_directive_searched): Remove.