Remove netdev_find_dev_by_in4

Remove netdev_find_dev_by_in4

netdev_find_dev_by_in4() appears to no longer be used and thus
can be removed. This also allows netdev_enumerate(), the
enumerate member of struct netdev_class and netdev_linux_enumerate()
to be removed.

I noticed this as netdev_linux_enumerate() makes use of if_nameindex()
and if_freenameindex() which are not available when compiling using
the Android NDK r6b (Android API level 13).