Remove Platform::supportsOutputChanges()

If the platform does not support configuring outputs, applying a config
must fail. However, almost all output backends support output
configuring except, perhaps, the x11 backend, but that case doesn't
matter.

src/backends/drm/drm_backend.cpp

@@ -85,7 +85,6 @@ DrmBackend::DrmBackend(Session *session, QObject *parent)
 {
     setSupportsPointerWarping(true);
     setSupportsGammaControl(true);
-    supportsOutputChanges();
 }
 
 DrmBackend::~DrmBackend() = default;

src/backends/virtual/virtual_backend.cpp

@@ -31,7 +31,6 @@ VirtualBackend::VirtualBackend(QObject *parent)
         }
     }
 
-    supportsOutputChanges();
     setSupportsPointerWarping(true);
     setSupportsGammaControl(true);
 }

src/backends/wayland/wayland_backend.cpp

@@ -576,7 +576,6 @@ WaylandBackend::WaylandBackend(QObject *parent)
     , m_connectionThreadObject(new ConnectionThread(nullptr))
     , m_connectionThread(nullptr)
 {
-    supportsOutputChanges();
     connect(this, &WaylandBackend::connectionFailed, qApp, &QCoreApplication::quit);
 
 #if HAVE_WAYLAND_EGL

src/platform.h

@@ -391,14 +391,6 @@ protected:
         m_supportsGammaControl = set;
     }
 
-    /**
-     * Whether the backend is supposed to change the configuration of outputs.
-     */
-    void supportsOutputChanges()
-    {
-        m_supportsOutputChanges = true;
-    }
-
 private:
     bool m_ready = false;
     QSize m_initialWindowSize;
@@ -409,7 +401,6 @@ private:
     EGLDisplay m_eglDisplay;
     EGLContext m_globalShareContext = EGL_NO_CONTEXT;
     bool m_supportsGammaControl = false;
-    bool m_supportsOutputChanges = false;
     CompositingType m_selectedCompositor = NoCompositing;
     Output *m_primaryOutput = nullptr;
 };