sessionService: use a generic removeAll function to unset FORM data

Instead of manually removing entries in the SESSION object, let the
SessionService do the work in creating poll controllers.

app/classes/Framadate/Services/SessionService.php

@@ -55,9 +55,20 @@ class SessionService {
         unset($_SESSION[$section][$key]);
     }
 
+    /**
+     * Remove a session section
+     *
+     * @param $section
+     */
+    public function removeAll($section) {
+        assert(!empty($section));
+
+        unset($_SESSION[$section]);
+    }
+
     private function initSectionIfNeeded($section) {
         if (!isset($_SESSION[$section])) {
             $_SESSION[$section] = [];
         }
     }
-} 
+} 

create_classic_poll.php

@@ -195,7 +195,7 @@ switch ($step) {
         }
 
         // Clean Form data in $_SESSION
-        unset($_SESSION['form']);
+        $sessionService->removeAll('form');
 
         // Delete old polls
         $purgeService->repeatedCleanings();

create_date_poll.php

@@ -230,7 +230,7 @@ switch ($step) {
         }
 
         // Clean Form data in $_SESSION
-        unset($_SESSION['form']);
+        $sessionService->removeAll('form');
 
         $purgeService->repeatedCleanings();