Remove leftover references to dwarf2_per_cu_data::dwarf2_per_objfile

Remove leftover references to dwarf2_per_cu_data::dwarf2_per_objfile

This patch removes the remaining references to that field in obvious
ways (the same object is already available some other way in these
contexts).

gdb/ChangeLog:

	* dwarf2/read.c (process_psymtab_comp_unit): Remove reference to
	dwarf2_per_cu_data::dwarf2_per_objfile.
	(compute_compunit_symtab_includes): Likewise.
	(dwarf2_cu::start_symtab): Likewise.

Change-Id: I965700fa793d8457711a2d6ae448aaefd779eb96