Don't trigger worker-thread cleanup when destruction has already started

[Snuffleupagus]

Note how we're currently skipping all main-thread cleanup when document destruction has started, but for some reason we're still dispatching the "Cleanup" message. This seems like a simple oversight, since destruction will already invoke the BasePdfManager.cleanup method (on the worker-thread) to fully clear-out all caches.

[Snuffleupagus]

Note in particular how the "Terminate" case triggers full clean-up, whereas it's only partial in the "Cleanup" case:

pdf.js/src/core/worker.js

Lines 792 to 806 in 9f5fd07

	handler.on("Cleanup", function wphCleanup(data) {
	return pdfManager.cleanup(/* manuallyTriggered = */ true);
	});
	handler.on("Terminate", function wphTerminate(data) {
	terminated = true;
	const waitOn = [];
	if (pdfManager) {
	pdfManager.terminate(new AbortException("Worker was terminated."));
	const cleanupPromise = pdfManager.cleanup();
	waitOn.push(cleanupPromise);

[Snuffleupagus]

/botio unittest

[pdfjsbot]

From: Bot.io (Linux m4)

---

Received

Command cmd_unittest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.241.84.105:8877/4ff99bc3ac2fcf1/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

---

Success

Full output at http://54.241.84.105:8877/4ff99bc3ac2fcf1/output.txt

Total script time: 3.21 mins

• Unit Tests: Passed

[Snuffleupagus]

/botio integrationtest

[pdfjsbot]

From: Bot.io (Linux m4)

---

Received

Command cmd_integrationtest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.241.84.105:8877/16b5304fe72cb7e/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

---

Success

Full output at http://54.241.84.105:8877/16b5304fe72cb7e/output.txt

Total script time: 4.40 mins

• Integration Tests: Passed