only sink window if actually fullscreened

`EwmhFullscreen` unconditionally sinks windows in response to a
`_NET_WM_STATE` root window message removing `_NET_WM_STATE_FULLSCREEN`.
Unfortunately, at least some versions of either the Gtk or GNOME
libraries send this on startup while restoring a window's last
known state, which means a `manageHook` `doFloat` will be undone.

This change ignores the remove if the window is not fullscreen.

A followup issue, xmonad#865, has been added for the follow-on problem
that the floating state of a window is not restored on removal of
the fullscreen state.

Fixes: xmonad#820

XMonad/Hooks/EwmhDesktops.hs

@@ -548,7 +548,7 @@ fullscreenEventHook'
     when (action == add || (action == toggle && not isFull)) $ do
       chWstate (fi fullsc:)
       windows . appEndo =<< runQuery fullscreenHook win
-    when (action == remove || (action == toggle && isFull)) $ do
+    when (isFull && (action == remove || action == toggle)) $ do
       chWstate $ delete (fi fullsc)
       windows . appEndo =<< runQuery unFullscreenHook win