Remove 'tools/install_venv.py' reference paragraph
Since 'tools/install_venv.py' has been removed during refactor[1].
This paragraph displayed in quickstart may cause some confusion to
someone who new to horizon, so remove this paragraph.
[1]https://github.com/openstack/horizon/commit/6529cd1eb8aef16b7ded96779c684644ed5a9fe0
@@ -179,10 +179,6 @@ be used in any Django project.
The ``openstack_dashboard`` directory contains a reference Django project that
uses ``horizon``.
-For development, both pieces share an environment which (by default) is
-built with the ``tools/install_venv.py`` script. That script creates a
-virtualenv and installs all the necessary packages.
-
If dependencies are added to either ``horizon`` or ``openstack_dashboard``,
they should be added to ``requirements.txt``.
