X-nay to the ind-bay: Remove BindHelpers.xbind, plus a couple of extra warnings

Main thing here is removing BindHelpers.xbind, which was deprecated. This removes the
functions deprecated by #1546 for 2.6 in Lift 3, and introduces the same alternatives introduced
in that PR into Lift 3. It contributes to #1534 .

It also fixes three more warnings, one missed one in lift-json and two javac warnings.

The only remaining warnings after this should be in `lift-testkit` and related to code that
should be removable.