Remove all_type_unit_groups, unused.

	* dwarf2read.c (struct dwarf2_per_objfile): Delete unused members
	n_type_unit_groups, all_type_unit_groups.  All uses removed.
	(get_type_unit_group, build_type_unit_groups): Delete forward decls.
	(dw2_get_cutu): Renamed from dw2_get_cu.  All callers updated.
	(dw2_get_cu): Renamed from dw2_get_primary_cu.  All callers updated.
	(add_type_unit_group_to_table): Delete.