Whilst working on removing AppContext from some datatransfer classes and looking at
a helper method in SwingUtilities2, I noticed in AWT Event classes some similar checks for
SystemClipboard access.
It appears that this could only be false in a headless environment, and previously
if a SecurityManager disallowed it.

But there also doesn't appear to be any code that does anything based on this except
copy the value around to event to event.

Since nothing uses it, I think we can just delete it. No tests failed with this deletion.