Remove dwarf2_per_cu_data::dwarf2_per_objfile

Remove dwarf2_per_cu_data::dwarf2_per_objfile

Nothing references this field anymore, remove it.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_per_cu_data):
	<dwarf2_per_objfile>: Remove.
	* dwarf2/read.c (create_cu_from_index_list): Don't assign
	dwarf2_per_objfile.
	(create_signatured_type_table_from_index): Likewise.
	(create_signatured_type_table_from_debug_names): Likewise.
	(create_debug_type_hash_table): Likewise.
	(fill_in_sig_entry_from_dwo_entry): Likewise.
	(create_type_unit_group): Likewise.
	(read_comp_units_from_section): Likewise.
	(create_cus_hash_table): Likewise.