Strengthen prop_applyBlocks_filteredTxs_someOurs.

The `applyBlocks` function was recently changed so that fees of incoming
transactions are no longer stripped away (i.e., no longer replaced with
`Nothing`).

This change was made in the following commit:

120c7de

Consequently, it is now more straightforward to test our expectations
about the `fee` field within our property tests for `applyBlocks`.

This commit strengthens the `prop_applyBlocks_filteredTxs_someOurs`
property, which tests the `applyBlocks` function in scenarios where
**_only some_** transactions in an arbitrary block sequence are
relevant to an arbitrary wallet.

In particular, we now test that `applyBlocks` does not modify the `fee`
field in any way, whereas before, the `fee` field was deliberately
excluded from the test.

lib/wallet/test/unit/Cardano/Wallet/Primitive/ModelSpec.hs

@@ -2659,10 +2659,7 @@ prop_applyBlocks_filteredTxs_someOurs
     -> Pretty BlockSeq
     -> Property
 prop_applyBlocks_filteredTxs_someOurs (Pretty someOurs) (Pretty blockSeq)
-    -- TODO: we currently ignore transaction fees, as under some circumstances
-    -- the 'applyBlocks' function can modify fee values:
-    = fmap (set #fee Nothing) ourTxsReturned ====
-      fmap (set #fee Nothing) ourTxsExpected
+    = ourTxsReturned ==== ourTxsExpected
     & labelInterval 10
         "length allTxsProvided"
         (length allTxsProvided)