Remove obsolete pkg-config variables
librsvg.pc.in defines the following variables, which I think are obsolete:
svgz_supported=true
css_supported=true
Both are unchanged since 2011. I couldn't find any users of those variables on codesearch.debian.net.
I think we can remove them safely. This would make it slightly easier to switch to cargo-c (see #552 (closed)) since it doesn't support extra pkg-config variables yet.