Avoid multiple identical hints

Summary:
The last three hints shows the steps of simplifying the radical and dividing by a common factor, but sometimes the radical can't be simplified and there is no common factor.
For https://khanacademy.atlassian.net/browse/AI-407546

Test Plan: quadratic_equation.html?debug&problem=split_C&seed=59 should have 7 different hints, quadratic_equation.html?seed=139&problem=split_C&debug&lang=en should have 5. You could also test the other exercise type for luck.

Reviewers: emily, eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D14753