Remove redundant use of <>

`returnProposalDeposits` is never called with duplicate proposals, if it
was we would have been in trouble, because then deposit would be
returned into the treasury multiple times, which would be equivalent to
minting ADA.

Considering that this case is impossible anyways it makes more sense to
use more efficient and safer `insert` instead of `insertWith (<>)`

eras/conway/impl/src/Cardano/Ledger/Conway/Rules/Epoch.hs

@@ -217,7 +217,7 @@ returnProposalDeposits removedProposals oldUMap =
               (RewDepUView um)
           , unclaimed
           )
-      | otherwise = (um, Map.insertWith (<>) (gasId gas) (gasDeposit gas) unclaimed)
+      | otherwise = (um, Map.insert (gasId gas) (gasDeposit gas) unclaimed)
     addReward c rd =
       -- Deposits have been validated at this point
       rd {rdReward = rdReward rd <> compactCoinOrError c}