This replaces make_cleanup_free_objfile with std::unique_ptr.

gdb/ChangeLog
2017-11-04  Tom Tromey  <tom@tromey.com>

	* objfiles.c (do_free_objfile_cleanup): Remove.
	* compile/compile-object-load.c (compile_object_load): Update.
	* objfiles.h (make_cleanup_free_objfile): Remove.