Make dwarf2_free_objfile static

I noticed that dwarf2_free_objfile can be made static, by changing it
to be a registry cleanup function.  This simplifies the code, as well,
because now symbol readers don't have to explicitly call it.

Tested by the buildbot.

gdb/ChangeLog
2018-06-28  Tom Tromey  <tom@tromey.com>

	* coffread.c (coff_symfile_finish): Update.
	* xcoffread.c (xcoff_symfile_finish): Update.
	* elfread.c (elf_symfile_finish): Update.
	* symfile.h (dwarf2_free_objfile): Don't declare.
	* dwarf2read.c (_initialize_dwarf2_read): Use
	register_objfile_data_with_cleanup.
	(dwarf2_free_objfile): Now static.  Change signature.