[chores] Use the method of User model for invalidation organization cache

[pandafy]

In #424 we added some code for invalidating a user's organizations cache. Once the model method gets added to openwisp-users for cache invalidation, we should update the following code to use that method:

openwisp-radius/openwisp_radius/saml/views.py

Line 59 in cf7497e

cache.delete('user_{}_organizations'.format(user.pk))

Depends On

• openwisp/openwisp-users#322