Use htab_up in auto-load.c

Use htab_up in auto-load.c

This changes auto-load.c to use htab_up, rather than manually calling
htab_delete.

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

	* auto-load.c (struct auto_load_pspace_info)
	<~auto_load_pspace_info, auto_load_pspace_info>: Remove.
	<loaded_script_files, loaded_script_texts>: Change type to
	htab_up.
	(~auto_load_pspace_info) Remove.
	(init_loaded_scripts_info, maybe_add_script_file)
	(maybe_add_script_text, auto_load_info_scripts): Update.