[api-minor] Stop sending "UnsupportedFeature" from the worker-thread GetOperatorList-handling

This code was added all the way back in PR 6698, almost seven years ago, for backwards compatibility reasons. At this point in time, it seems that we can remove that since:
 - We have more fine-grained "UnsupportedFeature" reporting elsewhere in the worker-thread code nowadays.
 - The GetOperatorList-handling is now using `ReadableStream`s, which means that errors are being forwarded to the main-thread anyway.
 - We're also no longer displaying a notification-bar, in the *built-in* Firefox PDF Viewer, for any of these "UnsupportedFeature" messages.

src/core/worker.js

@@ -27,7 +27,6 @@ import {
   stringToPDFString,
   UnexpectedResponseException,
   UnknownErrorException,
-  UNSUPPORTED_FEATURES,
   VerbosityLevel,
   warn,
 } from "../shared/util.js";
@@ -728,12 +727,6 @@ class WorkerMessageHandler {
               if (task.terminated) {
                 return; // ignoring errors from the terminated thread
               }
-              // For compatibility with older behavior, generating unknown
-              // unsupported feature notification on errors.
-              handler.send("UnsupportedFeature", {
-                featureId: UNSUPPORTED_FEATURES.errorOperatorList,
-              });
-
               sink.error(reason);
 
               // TODO: Should `reason` be re-thrown here (currently that casues