Combine the stringToUTF16String and stringToUTF16BEString helper functions

Given that these functions are virtually identical, with the latter only adding a BOM, we can combine the two. Furthermore, since both functions were only used on the worker-thread, there's no reason to duplicate this functionality in both of the `pdf.js` and `pdf.worker.js` files.

src/shared/util.js

@@ -1055,18 +1055,6 @@ function isAscii(str) {
   return /^[\x00-\x7F]*$/.test(str);
 }
 
-function stringToUTF16BEString(str) {
-  const buf = ["\xFE\xFF"];
-  for (let i = 0, ii = str.length; i < ii; i++) {
-    const char = str.charCodeAt(i);
-    buf.push(
-      String.fromCharCode((char >> 8) & 0xff),
-      String.fromCharCode(char & 0xff)
-    );
-  }
-  return buf.join("");
-}
-
 function stringToUTF8String(str) {
   return decodeURIComponent(escape(str));
 }
@@ -1198,7 +1186,6 @@ export {
   string32,
   stringToBytes,
   stringToPDFString,
-  stringToUTF16BEString,
   stringToUTF8String,
   TextRenderingMode,
   UnexpectedResponseException,