[r=dave-cheney] Remove thirdparty/pbkdf2
In preparation for the migration to github remove the private copy of pbkdf2.
It is probable that when this package was added, somewhere around 18 months ago, we did not have godeps or a established way of managing dependencies, so vendoring the package made sense at the time.
The only differences between our privat copy and the upstream is our inclusion of our note that the package is a fork.