mlxsw: spectrum_router: Make mlxsw_sp_router_fib4_del return void and remove warn

The function return value is not checked anywhere. Also, the warning
causes huge slowdown when removing large number of FIB entries which
were not offloaded, because of ordering issue. Ido's preparing
a patchset to fix the ordering issue, but that is definitelly not
net tree material.

Fixes: b45f64d1 ("mlxsw: spectrum_router: Use FIB notifications instead of switchdev calls")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>