objdump: Delete close optimisation

In commit cd6581da62c3, Nick made an optimisation that was reasonable
at the time, but then pr22032 came along and commit 7c0ed39626e3 made
bfd_close_all_done free memory.  So Nick's optimisation is now
ineffective, and the comment wrong.

	* objdump.c (display_file): Delete last_file param.  Update
	caller.  Call bfd_close always.