vrf: don't check for dstats and rth in uninit path

dstats and rth are always present because we fail the device registration
if they can't be allocated in vrf_init() (ndo_init) so drop the unnecessary
checks.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>