Use AbortController unconditionally with the Fetch API

Given the browsers that we currently support in the PDF.js project and the MDN compatibility data, see links below, it should no longer be necessary to check for the availability of `AbortController` before using it.
 - https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions#faq-support
 - https://developer.mozilla.org/en-US/docs/Web/API/AbortController#browser_compatibility

src/display/fetch_stream.js

@@ -35,7 +35,7 @@ function createFetchOptions(headers, withCredentials, abortController) {
   return {
     method: "GET",
     headers,
-    signal: abortController?.signal,
+    signal: abortController.signal,
     mode: "cors",
     credentials: withCredentials ? "include" : "same-origin",
     redirect: "follow",
@@ -114,9 +114,7 @@ class PDFFetchStreamReader {
       this._disableRange = true;
     }
 
-    if (typeof AbortController !== "undefined") {
-      this._abortController = new AbortController();
-    }
+    this._abortController = new AbortController();
     this._isStreamingSupported = !source.disableStream;
     this._isRangeSupported = !source.disableRange;
 
@@ -207,9 +205,7 @@ class PDFFetchStreamReader {
     if (this._reader) {
       this._reader.cancel(reason);
     }
-    if (this._abortController) {
-      this._abortController.abort();
-    }
+    this._abortController.abort();
   }
 }
 
@@ -224,10 +220,7 @@ class PDFFetchStreamRangeReader {
     this._readCapability = createPromiseCapability();
     this._isStreamingSupported = !source.disableStream;
 
-    if (typeof AbortController !== "undefined") {
-      this._abortController = new AbortController();
-    }
-
+    this._abortController = new AbortController();
     this._headers = createHeaders(this._stream.httpHeaders);
     this._headers.append("Range", `bytes=${begin}-${end - 1}`);
 
@@ -274,9 +267,7 @@ class PDFFetchStreamRangeReader {
     if (this._reader) {
       this._reader.cancel(reason);
     }
-    if (this._abortController) {
-      this._abortController.abort();
-    }
+    this._abortController.abort();
   }
 }