devel/developer-setup: Remove paragraph about mypy installation

Installation of mypy with the correct version is already handled
by the bin/install script.

docs/devel/developer-setup.rst

@@ -114,12 +114,6 @@ Checkout all the swh packages source repositories:
     (swh) ~/swh-environment$ pip install pre-commit
     (swh) ~/swh-environment$ ./bin/update
 
-Use the same mypy version our tox containers use:
-
-.. code-block:: console
-
-    (swh) ~/swh-environment$ pip install mypy==1.8.0
-
 In the future you can re-activate the created virtualenv with:
 
 .. code-block:: console