remove unnecessary logic for stylesheets

[jonschlinkert]

seemed cool when I was creating it, not it just looks like a PITA.

• https://github.com/assemble/assemble-swig-example/blob/master/Gruntfile.js#L30
• https://github.com/assemble/assemble-swig-example/blob/master/src/includes/navbar.hbs#L6