[BUGFIX] Ext:form - remove class attribute from flashMessages viewhelper

https://review.typo3.org/#/c/50285/ breaks the ext:form backend modul.
Remove "class" tag attributes from flashMessages viewHelper tags.

Resolves: #79306
Releases: master
Change-Id: I753cc5bf4deabfa824179c5e025044c65df4e069
Reviewed-on: https://review.typo3.org/51303


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>

typo3/sysext/form/Resources/Private/Backend/Layouts/FormManager.html

@@ -3,5 +3,5 @@
     includeCssFiles="{stylesheets}"
 />
 
-<f:flashMessages class="flashmessages" />
+<f:flashMessages />
 <f:render section="MainContent" />
\ No newline at end of file

typo3/sysext/form/Resources/Private/Frontend/Templates/Render.html

 {namespace formvh=TYPO3\CMS\Form\ViewHelpers}
-<f:flashMessages class="flashmessages" />
+<f:flashMessages />
 <f:if condition="{formConfiguration}">
     <formvh:render overrideConfiguration="{formConfiguration}"/>
 </f:if>
\ No newline at end of file