[GENERIC viewer] Re-factor the fileInput initialization

*This is yet another installment in a never-ending series of patches that attempt to simplify and improve old code.*

The `fileInput`-element is used to support the "Open file"-button in the `GENERIC` viewer, however this is very old code.
Rather than creating the element dynamically in JavaScript, we can simply define it conditionally in the HTML code thanks to the pre-processor. Furthermore, the `fileInput`-element currently has a number of unnecessary CSS rules, since the element is *purposely* never made visibly.

Note that with these changes, the `fileInput`-element will now *always* have `display: none;` set. This shouldn't matter, since we can still trigger the `click`-event of the element just fine (via JavaScript) and this patch has been successfully tested in both Mozilla Firefox and Google Chrome.

web/viewer.js

@@ -189,7 +189,10 @@ function getViewerConfiguration() {
     },
     errorWrapper,
     printContainer: document.getElementById("printContainer"),
-    openFileInputName: "fileInput",
+    openFileInput:
+      typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")
+        ? document.getElementById("fileInput")
+        : null,
     debuggerScriptPath: "./debugger.js",
   };
 }