Remove free_dwo_file_cleanup

Remove free_dwo_file_cleanup

This removes free_dwo_file_cleanup, the last cleanup in dwarf2read.c.
This is replaced with a unique_ptr; which, despite the fact that a
dwo_file is obstack-allocated, seemed like the best fit.

gdb/ChangeLog
2018-03-30  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (struct free_dwo_file_cleanup_data): Remove.
	(struct dwo_file_deleter): New.
	(dwo_file_up): New typedef.
	(open_and_init_dwo_file): Use dwo_file_up.
	(free_dwo_file_cleanup): Remove.