docs: don't include the type hierarchy

gtk-doc can't generate a type hierarchy when scanning gst-libav, and gtk-doc 1.30 onwards doesn't write a file if there is no type hierarchy (unlike previous releases, which wrote an empty file). This results in the build failing with gtk-doc 1.30 onwards, so remove the type hierarchy section from the documentation as it doesn't serve any purpose.

Fixes #57 (closed)