xml: T5862: drop defaultValue from mtu-68-16000.xml.i - use individual values

In order to lower the Ethernet default MTU we need to drop the common
defaultValue from the XML mtu-68-16000.xml.i building block. Per interface
default MTU is later overloaded by XML.

(cherry picked from commit e86761fa1307596c721c3ddf3a61d263e8f5177b)