minor #6702 Removed empty Notes on previous versions (mickaelandrieu)

This PR was submitted for the 3.1 branch but it was merged into the 2.8 branch instead (closes #6702).

Discussion
----------

Removed empty `Notes on previous versions`

On ``OptionsResolver`` component documentation, we have a ``Notes on previous versions`` section after Symfony 2.6.

Since Symfony 2.8 docs, this section is empty, so I suggest to remove it for version 3.x and probably re-add it in 2.8.

> This pull request should be applied in 3.0, 3.1, 3.2 **and 2.8** cf @wouterj comment

Commits
-------

892fcce Removed empty `Notes on previous versions`

components/options_resolver.rst

@@ -19,9 +19,6 @@ You can install the component in 2 different ways:
 
 .. include:: /components/require_autoload.rst.inc
 
-Notes on Previous Versions
---------------------------
-
 Usage
 -----