Fix FK error when deleting a user

Table "c_lp_category_user" (appeared while doing behat tests).

main/inc/lib/usermanager.lib.php

@@ -739,6 +739,12 @@ class UserManager
             TicketManager::deleteUserFromTicketSystem($user_id);
         }
 
+        $tableExists = $connection->getSchemaManager()->tablesExist(['c_lp_category_user']);
+        if ($tableExists) {
+            $sql = "DELETE FROM c_lp_category_user WHERE user_id = $user_id";
+            Database::query($sql);
+        }
+
         // Delete user from database
         $sql = "DELETE FROM $table_user WHERE id = '".$user_id."'";
         Database::query($sql);