[api-minor] Stop exposing the LoopbackPort class in the API

This was done all the way back in PR 8361, for a mozilla-central test that's since been removed. As can be seen in the following search results, there's no `LoopbackPort` invocation outside of the PDF.js code itself: https://searchfox.org/mozilla-central/search?q=LoopbackPort&path=

Given that the `LoopbackPort` is only used in connection with "fake workers", which is something that we don't officially recommend/support, this doesn't seem like functionality that we want to keep exposing in the public API.

src/pdf.js

@@ -43,7 +43,6 @@ import {
 import {
   build,
   getDocument,
-  LoopbackPort,
   PDFDataRangeTransport,
   PDFWorker,
   setPDFNetworkStreamFactory,
@@ -128,7 +127,6 @@ export {
   InvalidPDFException,
   isPdfFile,
   loadScript,
-  LoopbackPort,
   MissingPDFException,
   OPS,
   PasswordResponses,