COMP: Simplify MRML Scene API removing SceneModifiedTime methods.

Along with the methods "GetSceneModifiedTime" and "IncrementSceneModifiedTime",
the ivar SceneModifiedTime has been introduced in r5772.

Considering the Modified time of the scene is in fact returned by the
method "vtkMRMLScene::GetMTime()", having an other method named
"GetSceneModifiedTime" is rather confusing.

Since internally the method "GetSceneModifiedTime" was using the MTime
of the underlying node collection, the existing call to the method
have been changed into "GetNodes()->GetMTime()".

This commit is a follow-up of r18562 where the method "GetCurrentScene()"
has been renamed into "GetNodes()". For reference, the method
"GetCurrentScene()" has itself been introduced in r8315.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22673 3bd1e089-480b-0410-8dfb-8563597acbee