Remove function wrappers

This removes the wrapper functions and leaves librsvg-*.c empty except for the gtk-doc comments.

Seems to work for me, at least it passes the tests. But it definitely needs testing on other platforms before this gets merged.

Also we should discuss what to do about the gtk-doc comments. It would be much nicer if we could move them to the Rust code. Has that been done before, can gtk-doc handle this?