Remove Compositor created check

Summary:
Cleans up the Compositor code some more. The check is only
used in a single assert statement.

Test Plan: Compiles.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D22278

composite.h

@@ -143,16 +143,6 @@ public:
         return m_scene;
     }
 
-    /**
-     * @brief Checks whether the Compositor has already been created by the Workspace.
-     *
-     * This method can be used to check whether self will return the Compositor instance or @c null.
-     *
-     * @return bool @c true if the Compositor has been created, @c false otherwise
-     **/
-    static bool isCreated() {
-        return s_compositor != NULL;
-    }
     /**
      * @brief Static check to test whether the Compositor is available and active.
      *

thumbnailitem.cpp

@@ -41,7 +41,6 @@ AbstractThumbnailItem::AbstractThumbnailItem(QQuickItem *parent)
     , m_saturation(1.0)
     , m_clipToItem()
 {
-    Q_ASSERT(Compositor::isCreated());
     connect(Compositor::self(), SIGNAL(compositingToggled(bool)), SLOT(compositingToggled()));
     compositingToggled();
     QTimer::singleShot(0, this, SLOT(init()));