[TASK] Remove superfluous extraHidden variable from login template

Variable `extraHidden` was used in legacy pi-based plugins (which
have been removed in TYPO3 v11). Actually this variable was never
used in the modernized variant, based on Extbase and Fluid.

Resolves: #96815
Releases: main, 11.5
Change-Id: I63ef795fd8f44b0822eb081aa322549756706e14
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73386


Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>

typo3/sysext/felogin/Resources/Private/Templates/Login/Login.html

@@ -87,8 +87,6 @@
             <f:if condition="{noRedirect}!=''">
                 <f:form.hidden name="noredirect" value="1" />
             </f:if>
-
-            {extraHidden}
         </div>
     </fieldset>
 </f:section>