build: Make sassc mandatory and always generate CSS files from Sass sources

build: Make sassc mandatory and always generate CSS files from Sass sources

The previous implementation of the CSS generation logic considered sassc
an optional dependency and made sure for that reason that the result CSS
files ended up in the source directory, so that they could be checked
in and kept in sync with the source sass files.

As we are making sassc a required dependency, we can now stop doing that
extra work and simply rely on the CSS files being automatically generated
each time the sass sources change. By doing this, we can now effectively
get rid of the CSS files checked in the repo as well as of the parse_sass.sh
script, since the CSS files will now live on the build directory only.

https://bugzilla.gnome.org/show_bug.cgi?id=792822