QmlProfiler: Remove custom canvas implementation

The canvas integrated in QtQuick does the same thing. We can remove a
lot of code like this.

Change-Id: I6425ae4e1b542107defd9d76fa5755712a0f8613
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>