net: Remove '#ifdef CONFIG_NET_ROUTE' around forwarding getter and setter.

Because `forwarding` flag is available unconditionally the compilation
guard has no make sense. Additionally this flag and related getter and
setter can be used for e.g. IPv4 packets forwarding in future, which
would be impossible looking at `NET_ROUTE` dependencies.

Signed-off-by: Marcin Gasiorek <marcin.gasiorek@nordicsemi.no>