Removed the check for non-floating content

[fredriks123]

Removed the check for non-floating content before clearing PreviousContainer during LayoutRoot.CollectGarbage.

In the case of a detached content in a pane (content made floating), that content will already have been set as a child in a new floating window before LayoutRoot.CollectGarbage is called, thus not removing any possibly emptied panes.

[fredriks123]

#450