net: ipv6: Use correct API to remove router

net_if_ipv6_router_rm() is the correct API to remove IPv6 router
from the interface as it also cancels the timer and raises a mgmt
event. The net_if_router_rm() only sets router as unused and
nothing else.

Change-Id: I55114288c9ae748520b67b206edcd4f5e420b1af
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>