[cldoc][node] Removing import from nonexistent module

As mentioned in [this issue](https://github.com/jessevdk/cldoc/issues/18), installing and running the master image currently results in error when attempting to issue various commands.  This is because `clang/nodes/__init__.py` is importing from a module that isn't defined in the project.

This pull request is a single line change that removes this import statement.

I have tested this on my local development machine (64-bit ArchLinux), and after building a new `cldoc` with this changset applied, I now am able to generate documentation.