KSPM: Only clear firstChild mappings for top-level

Otherwise we would accidentally clear row 0 in top-level in the proxy
when some nested row 0 is removed in the source.

src/kselectionproxymodel.cpp

@@ -1263,8 +1263,10 @@ void KSelectionProxyModelPrivate::endRemoveRows(const QModelIndex &sourceParent,
         // If D and F are selected, the proxy contains E, F, G, H. If B is then deleted E to H will
         // be removed, including both first child mappings at E and G.
 
+        if (!proxyParent.isValid()) {
             removeFirstChildMappings(proxyStart, proxyEnd);
         }
+    }
 
     if (proxyParent.isValid()) {
         updateInternalIndexes(proxyParent, proxyEnd + 1, -1 * (proxyEnd - proxyStart + 1));