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.
Book/Doctrine Minor fix

Fixing bad copy/paste ("yaml" => "xml").
I'm checking/updating French doc so I see everything (or almost) ;)
removed unneeded configuration
Merge pull request #2637 from gregquat/patch-11

Book/Doctrine: Minor typo fix
Added documentation for GroupSequence
Fixed scheme in XML format
Typo in code example

SessionStorage should be NativeSessionStorage. Example fails to execute otherwise.
Improving the VCL examples
Fix doubled words
fix doubled words
fixed attribute name
Merge pull request #2631 from symfony/session-flash-revert

Revert "fixed example to check if the session exists when getting flash ...
Merge pull request #2641 from bamarni/patch-7

[Twig] removed unneeded configuration
reference/.../property_path : Minor fix, suggest to use 'mapped' option ...

... the explanation was not complete
Merge pull request #2647 from ClementGautier/2.0

[WIP] Added documentation for GroupSequence
[#2647] Minor tweaks to new validation group sequence docs
Merge pull request #2654 from ThijsFeryn/varnish_purge_fix

Varnish purge fix
Merge remote-tracking branch 'origin/2.1' into 2.1
Merge pull request #2652 from jalliot/patch-2

Fixed scheme in XML format
Merge pull request #2656 from Cangit/patch-2

Typo in code example