Revert "fixed example to check if the session exists when getting flash messages to avoid starting sessions when not needed"

It seems that there may be something stored in the flash, but you never know, because the session isn't started, so you never check.

This reverts commit a5168ad2f46264508b93b60c4c21a21384244662.
Merge pull request #2631 from symfony/session-flash-revert

Revert "fixed example to check if the session exists when getting flash ...