minor #8418 Stop to recommend FOSUser for Symfony 4 (dunglas)

This PR was submitted for the master branch but it was merged into the 4.0 branch instead (closes #8418).

Discussion
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Stop to recommend FOSUser for Symfony 4

It may be a bit controversial, but most contributors seems to agree that FOSUser isn't necessary anymore. Symfony has now builtin tools allowing to do what FOSUser provides in a more flexible way, and FOSUser itself generates a lot of support requests and dissatisfaction.

I propose to stop promoting FOSUser in official docs.

Commits
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d946686 Stop to recommend FOSUser for Symfony 4

Author: javiereguiluz

security/entity_provider.rst

@@ -12,15 +12,6 @@ you how to load your users from the database via a Doctrine entity.
 Introduction
 ------------
 
-.. tip::
-
-    Before you start, you should check out `FOSUserBundle`_. This external
-    bundle allows you to load users from the database (like you'll learn here)
-    *and* gives you built-in routes & controllers for things like login,
-    registration and forgot password. But, if you need to heavily customize
-    your user system *or* if you want to learn how things work, this tutorial
-    is even better.
-
 Loading users via a Doctrine entity has 2 basic steps:
 
 #. :ref:`Create your User entity <security-crete-user-entity>`