Remove morpheus priority, which broke custom themes. #312

This priority order cause that all morpheus stylesheets are loaded at very end of generated css file and custom theme rules are overwritten by morpheus. I tried morpheus without this rules and looks ok, but I'm quite not sure what's purpose of these priorities.

I removed two out of the three lines, see: https://github.com/piwik/piwik/commit/b06b1189c355ec8ecc9dc912f30da55432b9a126

(removing other line caused regressions)