[api-minor] Remove the postMessageTransfers parameter, and thus the ability to manually disable transferring of data, from the API

By transfering, rather than copying, `ArrayBuffer`s between the main- and worker-threads, you can avoid unnecessary allocations by only having *one* copy of the same data.
Hence manually setting `postMessageTransfers: false`, when calling `getDocument`, is a performance footgun[1] which will do nothing but waste memory.

Given that every reasonably modern browser supports `postMessage` transfers[2], I really don't see why it should be possible to force-disable this functionality.
Looking at the browser support, for `postMessage` transfers[2], it's highly unlikely that PDF.js is even usable in browsers without it. However, the feature testing of `postMessage` transfers is kept for the time being just to err on the safe side.

---
[1] This is somewhat similar to the, now removed, `disableWorker` parameter which also provided API users a much too simple way of reducing performance.

[2] See e.g. https://developer.mozilla.org/en-US/docs/Web/API/MessagePort/postMessage#Browser_compatibility and https://developer.mozilla.org/en-US/docs/Web/API/Transferable#Browser_compatibility

web/app_options.js

@@ -206,11 +206,6 @@ const defaultOptions = {
     value: false,
     kind: OptionKind.API,
   },
-  postMessageTransfers: {
-    /** @type {boolean} */
-    value: true,
-    kind: OptionKind.API,
-  },
   verbosity: {
     /** @type {number} */
     value: 1,