do not mutate the reserve as part of the fee balancing process

Everything in the past coin selection (without reserve) was easy to reason about. One could always look at the sum
of inputs and compare it with the sum of outputs and changes, and there were some nice properties about this. Having
mutable reserve makes it somewhat very confusing because, there's no trace of where the money comes from (turning
reserve into changes shouldn't make it disappear).

lib/core/src/Cardano/Wallet/Primitive/Fee.hs

@@ -229,17 +229,6 @@ rebalanceSelection opts s
     | φ_original == δ_original =
         (s, Fee 0)
 
-    -- some fee left to pay, and the reserve is non-empty, use it first.
-    | φ_original > δ_original && reserve s > Just (Coin 0) =
-        let
-            -- Safe because of the above guard.
-            Just (Coin r) = reserve s
-            r' = if r > φ_original
-                then Coin (r - φ_original)
-                else Coin 0
-        in
-            rebalanceSelection opts (s { reserve = Just r' })
-
     -- some fee left to pay, but we've depleted all change outputs
     | φ_original > δ_original && null (change s) =
         (s, Fee (φ_original - δ_original))
@@ -283,10 +272,7 @@ rebalanceSelection opts s
     δ_dangling = φ_original -- by construction of the change output
 
     extraChng = Coin (δ_original - φ_original)
-    sDangling = s
-        { change = splitChange extraChng (change s)
-        , reserve = Coin 0 <$ reserve s
-        }
+    sDangling = s { change = splitChange extraChng (change s) }
 
 -- | Reduce single change output by a given fee amount. If fees are too big for
 -- a single coin, returns a `Coin 0`.

lib/core/test/unit/Cardano/Wallet/Primitive/FeeSpec.hs

@@ -52,8 +52,6 @@ import Data.Functor.Identity
     ( Identity (runIdentity) )
 import Data.List.NonEmpty
     ( NonEmpty )
-import Data.Maybe
-    ( isNothing )
 import Data.Word
     ( Word64 )
 import Fmt
@@ -506,13 +504,11 @@ prop_rebalanceSelection sel onDangling = do
                 fee' /= Fee 0 || Fee (delta sel') == estimateFee opts sel'
             SaveMoney ->
                 fee' /= Fee 0 || Fee (delta sel') >= estimateFee opts sel'
-    let reserveIsEmpty =
-            case reserve sel of
-                Nothing -> isNothing (reserve sel')
-                Just{}  -> reserve sel' == Just (Coin 0)
+    let reserveIsUnchanged =
+            reserve sel == reserve sel'
     conjoin
         [ property selectionIsBalanced
-        , property reserveIsEmpty
+        , property reserveIsUnchanged
         ]
         & counterexample (unlines
             [ "selection (before):", pretty sel