Sign natively on Windows

* Replace the windows signing script with certificateSubjectName
   option which finds the right certificate from the smart card
 * Remove the windows signing script as electron-builder does it
   out-of-the-box on windows
 * Remove the cert file which isn't necessary with native signing

package.json

@@ -79,7 +79,7 @@
           "ia32"
         ]
       },
-      "sign": "scripts/electron_winSign"
+      "certificateSubjectName": "New Vector Ltd"
     },
     "directories": {
       "output": "dist"