elf: unload _used_by_resolve_plt_got on object removal

Like _needed, _used_by_resolve_plt_got should be cleared as well
when the underlying object is being removed.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>