Fix Gtk3 crash when running inside of IPython

[mdboom]

As reported in #2624, the Gtk3 backends and IPython are not playing well together, and either no window is displayed, or Python exceptions or even segfaults are thrown.

This, along with ipython/ipython#4640 seem to make everyone happy.

[tacaswell]

Sorry for the poor issue reporting on this.

I don't know what you think the likely timeline on #2624 is, but would it be possible to hold off on merging this until after that one (just to avoid a re-base where I have to think).

[mdboom]

I think we're likely to have a complicate merge and/or rebase regardless, since this is on v1.3.x...

[efiring]

Given that this is a 1-line deletion, I don't see a major problem. One way to handle it would be to merge this in 1.3.x, and then just make the equivalent change in the refactor PR.

[tacaswell]

Ah, I missed what branch it was against.

The problem is that the change is now in a different file which, from re-basing to get rid of the QT4Agg functions, I found git does not deal with super gracefully. As Eric says this won't involve too much thinking, ignore my complaints.