Remove scoped_inc_dec_ref

Remove scoped_inc_dec_ref

We can remove scoped_inc_dec_ref by changing the sole user to instead
keep a vector of thread_info_ref objects.  This removes some manual
reference counting and simplifies the code a bit.

gdb/ChangeLog
2020-12-11  Tom Tromey  <tom@tromey.com>

	* thread.c (class scoped_inc_dec_ref): Remove.
	(tp_array_compar_ascending, tp_array_compar_descending): Change
	parameter types.
	(thread_apply_all_command): Use thread_info_ref.