wallet: Revert input selection post-pruning
This reverts PR #4906, "Coinselection prunes extraneous inputs from
ApproximateBestSubset".
Apparently the previous behavior of slightly over-estimating the set of
inputs was useful in cleaning up UTXOs.
See also #7664, #7657, as well as 2016-07-01 discussion on #bitcoin-core-dev IRC.