We removed support in the code ages ago, but overlooked this part of the documentation. Also remove some dead code in Xvnc on the same theme.
This allows Xvnc to build with xorg-server 1.20.7, which requires OS layers to implement a ddxInputThreadInit function when configured with --enable-input-thread (the default). relevant xorg-server commit: e3f26605d85d987da434640f52646d728f1fe919