Remove custom stylesheet, fall back to default because qmi isn't compatible with Doxygen 1.8.7

A fix for numenta/nupic.tools#70 in `nupic.core`, there should also be one in `nupic`.

This is a fallback, not that pleasing to the eye as [qmi](https://github.com/skozlovf/doxygen-qmi-style), but it's compatible and doesn't have icons missing. This is considered a temporal but possible long-run solution because `qmi` is no longer maintained and requires quite some tweak.

Also note that `HTML_STYLESHEET` is deprecated by `HTML_EXTRA_STYLESHEET` and the latter do style monkey-patching and more compatible with future versions of Doxygen.