remove configuration option 'mtu'

This isn't useful because anyone wanting to set the IPv6 MTU
can simply set that by setting the IPv6 MTU on the interface.

Test: builds
Bug: 144730808
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Id140dc9865d33b66b120b932cb16e4d4fa70fc47
1 file changed