Remove partial symbol statistics

The "n_psyms" statistic in the per-objfile stats is not really needed,
but its use requires passing the objfile to add_psymbol.  This patch
removes the field in favor of counting the psyms when needed.

Note that this is not exactly equivalent -- in the old approach, a
psymbol can in theory be created and then the psymtab discarded, which
would increment the counter.  This does not seem very important to me.

I rewrote the code to count partial symbols; though TBH I think that
this information is not really very useful.

gdb/ChangeLog
2020-11-01  Tom Tromey  <tom@tromey.com>

	* symmisc.c (count_psyms): New function.
	(print_objfile_statistics): Use it.
	* psymtab.c (append_psymbol_to_list): Remove.
	(partial_symtab::add_psymbol): Inline append_psymbol_to_list.
	* objfiles.h (struct objstats) <n_psyms>: Remove.

gdb/objfiles.h

@@ -194,9 +194,6 @@ struct obj_section
 
 struct objstats
 {
-  /* Number of partial symbols read.  */
-  int n_psyms = 0;
-
   /* Number of full symbols read.  */
   int n_syms = 0;