sway: fix "Mapping is not an object" warning

Fixes #3763.

Also a little code simplications while we're at it.

include/modules/sway/workspaces.hpp

@@ -49,7 +49,6 @@ class Workspaces : public AModule, public sigc::trackable {
   std::vector<std::string> workspaces_order_;
   Gtk::Box box_;
   std::string m_formatWindowSeperator;
-  std::string m_windowRewriteDefault;
   util::RegexCollection m_windowRewriteRules;
   util::JsonParser parser_;
   std::unordered_map<std::string, Gtk::Button> buttons_;