It was deprecated in Newton timeframe. Now we just clean it up from the
tree.
DocImpact: Any advertise_mtu option notions in documentation should be
removed.
UpgradeImpact: After upgrade, all DHCPv4 subnets will see the MTU option
served via corresponding DHCPv4 option. Also, all IPv6 subnets connected
to routers will see MTU set in Router Advertisement messages.
NeutronLibImpact: This patch will break any 3party plugins that directly
access the configuration option.
Change-Id: I31e15018fe764de0fe4d6de7da3c1d9f2cc1d532