Remove the "require spree/frontend" directive from the main css.

Spree applications that use different fronted stylesheets cannot use spree_static_content without including the spree/frontend css, this can ruin their whole layout (for example: a Spree app that uses the [spree_bootstrap_frontend](https://github.com/200Creative/spree_bootstrap_frontend) extension). 
I think this can be safely removed because spree/frontend is included by default in every Spree app.