Remove clear_symtab_users_cleanup

Remove clear_symtab_users_cleanup

This removes clear_symtab_users_cleanup, replacing it with uses of
forward_scope_exit.

gdb/ChangeLog:
2019-01-23  Tom Tromey  <tom@tromey.com>
	    Pedro Alves  <palves@redhat.com>

	* symfile.c: Include forward-scope-exit.h.
	(clear_symtab_users_cleanup): Replace forward declaration with
	a FORWARD_SCOPE_EXIT.
	(syms_from_objfile_1): Use the forward_scope_exit and
	gdb::optional instead of cleanup_function.
	(reread_symbols): Use the forward_scope_exit instead of
	cleanup_function.
	(clear_symtab_users_cleanup): Remove function.