Use htab_up in filename_seen_cache

Use htab_up in filename_seen_cache

This changes filename_seen_cache to use htab_up, rather than explicit
calls to htab_delete.

gdb/ChangeLog
2020-09-17  Tom Tromey  <tom@tromey.com>

	* filename-seen-cache.c (filename_seen_cache::filename_seen_cache)
	(filename_seen_cache::clear): Update.
	(~filename_seen_cache): Remove.
	(filename_seen_cache::seen): Update.
	* filename-seen-cache.h (class filename_seen_cache) <m_tab>: Now
	htab_up.
	<~filename_seen_cache>: Remove.
	<traverse>: Update.