Remove check that preferred lifetime >= t2

For a single address, DHCPv6 won't work well (i.e. potentially lose provisioning) if
the preferred lifetime is lower than t2. However, this might be a valid
scenario in a renumbering event in the presence of multiple prefixes.

Additionally, in DHCPv6 it is up to the server to configure the client
correctly.

Test: TH
Change-Id: I37b2a0b89deda85b3f7be29c8f02f685aa6e65f8