Fix two out-of-date comments

Fix two out-of-date comments

While looking at psymtabs again, I noticed a couple of outdated
comments.  These days, psymtabs can be destroyed, as they are no
longer obstack-allocated.

gdb/ChangeLog
2020-08-28  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (xcoff_end_psymtab): Update comment.
	* dbxread.c (dbx_end_psymtab): Update comment.