fix(swingset): we no longer need bigintJSONStringify for transcripts

because #2429 changed transcripts stop including `crankNumber`, which is a
Nat (and therefore now a BigInt).

packages/SwingSet/src/kernel/state/vatKeeper.js

@@ -234,16 +234,6 @@ export function makeVatKeeper(
     }
   }
 
-  // TODO: use marshal or some other library for handling the
-  // stringification of BigInts
-  function bigintJSONStringify(x) {
-    return JSON.stringify(
-      x,
-      (_key, value) => (typeof value === 'bigint' ? value.toString() : value),
-      // return everything else unchanged
-    );
-  }
-
   /**
    * Append a message to the vat's transcript.
    *
@@ -252,7 +242,7 @@ export function makeVatKeeper(
   function addToTranscript(msg) {
     const id = Nat(BigInt(storage.get(`${vatID}.t.nextID`)));
     storage.set(`${vatID}.t.nextID`, `${id + 1n}`);
-    storage.set(`${vatID}.t.${id}`, bigintJSONStringify(msg));
+    storage.set(`${vatID}.t.${id}`, JSON.stringify(msg));
   }
 
   function vatStats() {