Don't debit deposits pot twice on stake refund (#299)

[sandtreader]

One-line deletion! Fixes #299

This is an interesting one - it turns out although there was the mechanism to post stake refunds to the end of epoch, it was never actually actioned until @lowhung rightly added it in 497214e:

             self.pots.deposits -= deposit;
 
+            // Schedule refund
+            self.stake_refunds.push((stake_address.clone(), deposit));
+

So we were previously only debiting the deposits pot, but not actually paying to the reward account - but adding this meant debiting the pot twice, but does pay the reward. So we gained one, lost one. Now we gain both :-)