Remove a test which doesn't test what it intends to

Dates are in fact JSON-serializable, but as strings.

Recent changes to `isMessageEqual` mean that the `JSON.stringify` calls
cannot fail, because the `isMessage` check ensures all the fields we
serialize are strings. Writing a test to check for this failure mode is
not possible. There is a refactoring hazard here, so I added a comment
to call this out.

src/shared/port-util.js

@@ -42,14 +42,12 @@ export function isMessageEqual(data, message) {
     return false;
   }
 
-  try {
+  // We assume `JSON.stringify` cannot throw because `isMessage` verifies that
+  // all the fields we serialize here are serializable values.
   return (
     JSON.stringify(data, Object.keys(message).sort()) ===
     JSON.stringify(message, Object.keys(message).sort())
   );
-  } catch {
-    return false;
-  }
 }
 
 /**

src/shared/test/port-util-test.js

@@ -95,15 +95,6 @@ describe('port-util', () => {
         expectedResult: false,
         reason: 'data has one property that is different',
       },
-      {
-        data: {
-          frame1: new Date(), // not JSON-serializable
-          frame2,
-          type,
-        },
-        expectedResult: false,
-        reason: "data has one property that can't be serialized",
-      },
     ].forEach(({ data, expectedResult, reason }) => {
       it(`returns '${expectedResult}' because the ${reason}`, () => {
         const result = isMessageEqual(data, {