Remove old matrix experiment files.

In the qhints experiments (defined_and_undefined_matrix_operations.html, matrix_dimensions.html), choose the qhints alternatives. They performed marginally better, but not by a statistically significant amount.

In the video hint vs. solution only vs. steps only vs. traditional hints, choose traditional hints, with the goal of eventually extracting out the text into subhints. The steps only alternative performed better than the other alternatives in the new proficiency experiment, and roughly tied for best performance in the first review experiment (not sure if statistically significant). However, I suspect that some users learn better from having explanations in text, so having the text available in subhints sounds like the right way to go.

Auditors: eater, alpert