rm ext module- wait for new one

Author: jessica-mitchell

doc/htmldoc/index.rst

@@ -147,4 +147,3 @@ How NEST works --- The Big Picture
    NESTML <https://nestml.readthedocs.io/en/latest/>
    NESTGPU <https://nest-gpu.readthedocs.io/en/latest/>
    NEST NEAT <https://nest-neat.readthedocs.io/en/latest/>
-   NEST extension module <https://nest-extension-module.readthedocs.io/en/latest/>