removing duplicate code for navigation toggle on mobile

[rclations]

fixes #1166

The code removed here was duplicating the work in L166-215, causing issues on iOS mobile.

To reproduce, use Chrome developer tools device emulation for iPhone and test secondary navigation opens on the main navigation menu.

Previously, on click, the js would both add and remove the open class, appearing to do nothing for the end user. This removed the additional code so the open class is added and removed where appropriate.

[benlk]

Tested in the iOS simulator; it works there.

We don't have any notes in the original issue remarking on whether or not this affected the iOS simulator; it did not affect Chrome on Android at the time that I commented on that issue.

Do we want to try this in production, so it can be tested against actual iDevices instead of emulated ones?

[rclations]

good idea - I just loaded this up on our umbrella staging, and it seems to be working