unlet TreeDirNode.openInNewTab since we are overriding it

Older versions of vim require you to unlet a dictionary function if you
are replacing it (i.e. overriding it in a prototype OO scenario). The
unlet call got lost in the recent refactoring.

plugin/NERD_tree.vim

@@ -1688,6 +1688,7 @@ function! s:TreeDirNode.openExplorer()
     call self.open({'where': 'p'})
 endfunction
 "FUNCTION: TreeDirNode.openInNewTab(options) {{{3
+unlet s:TreeDirNode.openInNewTab
 function! s:TreeDirNode.openInNewTab(options)
     call s:deprecated('TreeDirNode.openInNewTab', 'is deprecated, use open() instead')
     call self.open({'where': 't'})