gui: use the stored CSubNet entry when unbanning

The previous code visualized the `CSubNet` object as string, then
parsed that string back to `CSubNet`. This is sub-optimal given that
the original `CSubNet` object can be used directly instead.

This avoids calling `LookupSubNet()` from the GUI.

Co-authored-by: furszy <matiasfurszyfer@protonmail.com>

src/qt/bantablemodel.cpp

@@ -178,3 +178,9 @@ bool BanTableModel::shouldShow()
 {
     return priv->size() > 0;
 }
+
+bool BanTableModel::unban(const QModelIndex& index)
+{
+    CCombinedBan* ban{static_cast<CCombinedBan*>(index.internalPointer())};
+    return ban != nullptr && m_node.unban(ban->subnet);
+}