net/ip: Removing useless extra data capability in neighbors

This is not used by anyone, and is unlikely to be useful actually.
Helps to save 4 bytes for each instance of struct net_nbr also (removing
a 2 bytes attributes, which was anyway generating a 4bytes loss due to
structure misalignment).

Removing relevant useless functions related to it as well.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>