Refactoring: Remove LegacyPromise

[bthorben]

All LegacyPromises should be refactored into native promises.

Optional: As this refactoring requires some thought anyway, it's probably a good idea to also make sure that all uses promises are actually needed. An example of doubtful usage would be in worker.js on line 276 inside wphSetupGetPage

[yurydelendik]

#4700 creates PDFJS.createPromiseCapability(), which returns object with promise, resolve, and reject.

Let's try these rules/patterns, if this will not work, let's change that:

1. async functions shall return promise (never capability or resolve/reject methods):

   function example() {
     return new Promise(function (resolve) {
       var request = requestSomething();
       request.onload = function (x) { resolve(x * 5); });
     });
   }
   

   or

   function example() {
      return Promise.resolve(5 * 5);
   }
   

2. functions shall create PromiseCapability only with goals to store it as a private object field or pass to the function:

   function ExampleObject() {
     this._capability = createPromiseCapability();
   }
   ExampleObject.prototype = {
     get promise() {
       return this._capability.promise;
     }
   }
   

   or

   function example() {
     var capability = createPromiseCapability();
     requestSomething(capability);
     return capability.promise;
   }