dev-env: Remove locustio from dependencies and nix-shell

setup.py

@@ -116,7 +116,6 b' test_requirements = ['
     'cssselect',
     'flake8',
     'lxml',
-    'locustio',
     'mock',
     'pytest',
     'pytest-cov',

shell.nix

@@ -45,7 +45,6 b' in enterprise.override (attrs: {'
       bumpversion
       invoke
       ipdb
-      locustio
     ]);
 
   shellHook = attrs.shellHook +