Fixed performance bug in make_otu_heatmap_html.py.

Also removed a lot of old commented-out code. Fixes #731.

Tested this out on the OTU table where the performance bug was first discovered (in [this QIIME forum thread](https://groups.google.com/d/topic/qiime-forum/pWjJwLykAsk/discussion)). The script now runs in 14 seconds.