Remove Lease::t1_ and t2_ members

Currently our Lease class has members for t1_ and t2_, however we never actually use them other than in a few system tests. These values are not really specific to a given lease, and in fact, for v6 all leases share a common value through their IA_XX. While implementing #365 (closed) for v6 I spent a good deal of time in a rabbit hole chasing down why these members exist.

I suggest we remove them to avoid confusion and to any temptation hook developers might have to use them for something.