Switching to non-SoS kernel does not hide the language dropdown

[BoPeng]

@choran reported this from #72

How to reproduce

1. Start a sos notebook
2. Change kernel to R
3. The language dropdown stays.
4. Insert a new cell, the language dropdown is still created.

Even if it is difficult to remove dropdown menu from existing cell toolbars, new cells should not have them.

[Ichoran]

I can confirm that this solves the issue for me: new cells don't get the dropdown, and old ones fix themselves once you switch between cells. (In both directions: SoS to non-SoS removes the menu, while non-SoS to SoS adds it back.)