libnvdimm, badrange: remove a WARN for list_empty

Now that we're reusing the badrange functions for nfit_test, and that
exposes badrange injection/clearing to userspace via the DSM paths, it
is plausible that a user may call the clear DSM multiple times. Since it
is harmless to do so, we can remove the WARN in badrange_forget.

Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>