Remove rescan workaround for dcrwallet bug

[jholdstock]

decred/dcrwallet#1740 is a bug which results in unspendable outputs if the block were originally included in gets re-orged. This is a problem for dcrpool because it needs to be able to spend received coinbases in order to pay dividends to users who contribute mining power.

To work around the problem, a bunch of code was added in #324 which will trigger a wallet rescan if an unspendable coinbase is detected. This is less than ideal and should really be removed once the dcrwallet bug is fixed.