Remove a comment relating to CryptoJS

Preparation for #1239 (removing CryptoJS dependency).

src/common/lib/util/utils.ts

@@ -466,9 +466,9 @@ export const randomString = (numBytes: number): string => {
     (Platform.Config.getRandomValues as Function)(uIntArr);
     return Platform.BufferUtils.base64Encode(uIntArr);
   }
-  /* Old browser; fall back to Math.random. Could just use a
-   * CryptoJS version of the above, but want this to still work in nocrypto
-   * versions of the library */
+  /* No secure random generator available; fall back to Math.random.
+   * TODO we should no longer end up in this scenario — and hence should be able to remove this code — given that all supported platforms should now have a random generator — see https://github.com/ably/ably-js/issues/1332
+   */
   const charset = Platform.BufferUtils.base64CharSet;
   /* base64 has 33% overhead; round length up */
   const length = Math.round((numBytes * 4) / 3);