[mempool] clear mapDeltas entry if prioritisetransaction sets delta to 0

It's unnecessary to keep the data around, as it doesn't do anything. If
prioritisetransaction is called again, we'll make a new entry in
mapDeltas.

These entries are only deleted when the transaction is mined or conflicted
from a block (i.e. not in replacement or eviction), are persisted in
mempool.dat, and never expire. If node operators use the RPC to
regularly prioritise/de-prioritise transactions, these (meaningless)
map entries may hang around forever and take up valuable mempool memory.

src/txmempool.cpp

@@ -870,8 +870,17 @@ void CTxMemPool::PrioritiseTransaction(const uint256& hash, const CAmount& nFeeD
             }
             ++nTransactionsUpdated;
         }
+        if (delta == 0) {
+            mapDeltas.erase(hash);
+            LogPrintf("PrioritiseTransaction: %s (%sin mempool) delta cleared\n", hash.ToString(), it == mapTx.end() ? "not " : "");
+        } else {
+            LogPrintf("PrioritiseTransaction: %s (%sin mempool) fee += %s, new delta=%s\n",
+                      hash.ToString(),
+                      it == mapTx.end() ? "not " : "",
+                      FormatMoney(nFeeDelta),
+                      FormatMoney(delta));
+        }
     }
-    LogPrintf("PrioritiseTransaction: %s fee += %s\n", hash.ToString(), FormatMoney(nFeeDelta));
 }
 
 void CTxMemPool::ApplyDelta(const uint256& hash, CAmount &nFeeDelta) const