Allow expand_symtabs_matching to examine imported psymtabs

Allow expand_symtabs_matching to examine imported psymtabs

Currently the psymtab variant of expand_symtabs_matching has this
check:

      /* We skip shared psymtabs because file-matching doesn't apply
	 to them; but we search them later in the loop.  */
      if (ps->user != NULL)
	continue;

In a larger series I'm working on, it's convenient to remove this
check.  And, I noticed that a similar check is not done for
expand_symtabs_with_fullname.  So, it made sense to me to remove the
check here as well.

gdb/ChangeLog
2021-03-26  Tom Tromey  <tom@tromey.com>

	* psymtab.c (psymbol_functions::expand_symtabs_matching): Remove
	"user" check.