[docs] Updated social login instructions

pip install django-allauth[socialaccount] is not required anymore.

docs/user/social_login.rst

@@ -39,22 +39,12 @@ the user is signing-in with a social network.
 Setup
 -----
 
-Install ``django-allauth`` in the python environment used by OpenWISP:
-
-::
-
- # by default, in instances deployed
- # via ansible-openwisp2, the python env
- # is in /opt/openwisp2/env/
- source /opt/openwisp2/env/bin/activate
-
- pip install django-allauth[socialaccount]
+Ensure the your project ``settings.py`` contains the instructions shown in
+the example below, which shows how to configure the Facebook social login
+provider.
 
 .. include:: /partials/settings-note.rst
 
-Ensure your ``settings.py`` looks like the following (we will show how to
-configure of the Facebook social provider):
-
 .. code-block:: python
 
  INSTALLED_APPS = [