Remove help button from symbollevelv2 dialog

There is no help text available for this dialog and the button does nothing at
all at the moment

Author: m-kuhn

src/ui/qgssymbollevelsv2dialogbase.ui

@@ -47,7 +47,7 @@
       <enum>Qt::Horizontal</enum>
      </property>
      <property name="standardButtons">
-      <set>QDialogButtonBox::Cancel|QDialogButtonBox::Help|QDialogButtonBox::Ok</set>
+      <set>QDialogButtonBox::Cancel|QDialogButtonBox::Ok</set>
      </property>
     </widget>
    </item>