Make delistPools replace the set of delisted pools.

We must replace the set of delisted pools, rather than augmenting it.

In response to review feedback:

#2277 (comment)

Author: jonathanknowles

lib/core/src/Cardano/Pool/DB/Model.hs

@@ -432,7 +432,7 @@ mRollbackTo ti point = do
         | otherwise = Nothing
 
 mDelistPools :: [PoolId] -> ModelOp ()
-mDelistPools = modify #delisted . Set.union . Set.fromList
+mDelistPools = modify #delisted . const . Set.fromList
 
 mReadDelistedPools :: ModelOp [PoolId]
 mReadDelistedPools = Set.toList <$> get #delisted

lib/core/src/Cardano/Pool/DB/Sqlite.hs

@@ -491,8 +491,9 @@ newDBLayer trace fp timeInterpreter = do
             deleteWhere [ BlockSlot >. point ]
             -- TODO: remove dangling metadata no longer attached to a pool
 
-        delistPools =
-            insertMany_ . fmap PoolDelistment
+        delistPools pools = do
+            deleteWhere ([] :: [Filter PoolDelistment])
+            insertMany_ $ fmap PoolDelistment pools
 
         readDelistedPools =
             fmap (delistedPoolId . entityVal) <$> selectList [] []