remove MCompositeManagerPrivate::exposeDesktop()
remove MCompositeManagerPrivate::exposeDesktop()

It's not used since 17c519aaa5e0b38bce97c40aa723e46e5dbbe4c3.

nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,siraj/mcompositor,siraj/mcompositor,nemomobile-graveyard/mcompositor,siraj/mcompositor,nemomobile-graveyard/mcompositor,siraj/mcompositor