DOC: Remove mention of requirements-dev.txt

The requirements-dev.txt file has been removed, so mention of it should be removed from documentation.

Author: aylward

docs/development.md

@@ -7,7 +7,6 @@ Setup your system for development:
 ```bash
 git clone https://github.com/InsightSoftwareConsortium/itkwidgets.git
 cd itkwidgets
-pip install -r requirements-dev.txt
 pip install -e .
 pytest
 pytest --nbmake examples/*.ipynb