Fix qty doubling when consolidating towards a zero qty. lot (take 2)

This was already reported (#678) and fixed (51385e1).

The remaining bug is that if you never ever changed any of the merge
options, the "isQuantityEnabled" flag was ignored, because it was set
too late.

Closes: #705

src/desktop/selectmergemode.cpp

@@ -32,6 +32,9 @@ void SelectMergeMode::setQuantityEnabled(bool enabled)
     if (enabled != m_quantityEnabled) {
         m_quantityEnabled = enabled;
 
+        if (!enabled)
+            m_modes.remove(DocumentModel::Quantity);
+
         for (QButtonGroup *fieldGroup : std::as_const(m_fieldGroups)) {
             const auto fields = fieldGroup->property("documentFields").value<QVector<DocumentModel::Field>>();