Enable ESLint rules that no longer need to be disabled on a directory/file-basis

Given that browsers/environments without native support for both arrow functions and object shorthand properties are no longer supported in PDF.js, please refer to the compatibility information below, we can now enable a fair number of ESLint rules and also simplify/remove some `.eslintrc` files.

With the exception of the `no-alert` cases, all code changes were made automatically by using `gulp lint --fix`.

 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions#browser_compatibility
 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer#browser_compatibility

examples/webpack/main.js

@@ -25,7 +25,7 @@ loadingTask.promise
       var ctx = canvas.getContext("2d");
       var renderTask = pdfPage.render({
         canvasContext: ctx,
-        viewport: viewport,
+        viewport,
       });
       return renderTask.promise;
     });