fix a bug when opening files in a new tab

On bufleave we save the nerdtree window state. However, sometimes when
bufleave is triggered the tree is no longer open in this tab - e.g. for
secondary trees or mappings that open in another tab. Dont bother saving
screen state in these cases as the code doing the saving assumes the
tree is available in this tab.

lib/nerdtree/ui.vim

@@ -325,13 +325,6 @@ endfunction
 "Saves the current cursor position in the current buffer and the window
 "scroll position
 function! s:UI.saveScreenState()
-
-    "FIXME: b:NERDTreeType should really me moved to b:NERDTree.type. Update
-    "this when that refactor is done
-    if b:NERDTreeType == "secondary"
-        return
-    endif
-
     let win = winnr()
     call g:NERDTree.CursorToTreeWin()
     let self._screenState = {}