xwayland: Drop unused method

Xwayland::self() is not used anywhere.

xwl/xwayland.cpp

@@ -60,24 +60,15 @@ namespace KWin
 namespace Xwl
 {
 
-Xwayland *s_self = nullptr;
-
-Xwayland *Xwayland::self()
-{
-    return s_self;
-}
-
 Xwayland::Xwayland(ApplicationWaylandAbstract *app, QObject *parent)
     : XwaylandInterface(parent)
     , m_app(app)
 {
-    s_self = this;
 }
 
 Xwayland::~Xwayland()
 {
     stop();
-    s_self = nullptr;
 }
 
 QProcess *Xwayland::process() const

xwl/xwayland.h

@@ -27,8 +27,6 @@ class Xwayland : public XwaylandInterface
     Q_OBJECT
 
 public:
-    static Xwayland *self();
-
     Xwayland(ApplicationWaylandAbstract *app, QObject *parent = nullptr);
     ~Xwayland() override;