build: Require sassc for classic style

Meson has a strict separation of source- and build directory, and expects anything generated in the latter. That means that in order to maintain our current setup - shipping the generated CSS in the repo while also optionally updating it automatically when sassc is found - we have to fight the build system to some extent, which makes it less reliable than we would like.

Since we switched to sassc which is a more acceptable build-time dependency than the original ruby-based tool, just drop the CSS from the repo and unconditionally generate it from SASS if classic mode is enabled.