remove NERDTree.previousBuf() and related code

We were using this when closing a wintree - to go to the previous
buffer. Not sure why we weren't just using `:bprev` ...

lib/nerdtree/creator.vim

@@ -92,14 +92,11 @@ function! s:Creator.createWindowTree(dir)
     "we want the directory buffer to disappear when we do the :edit below
     setlocal bufhidden=wipe
 
-    let previousBuf = expand("#")
-
     "we need a unique name for each window tree buffer to ensure they are
     "all independent
     exec g:NERDTreeCreatePrefix . " edit " . self._nextBufferName()
 
     call self._createNERDTree(path, "window")
-    let b:NERDTree._previousBuf = bufnr(previousBuf)
     call self._setCommonBufOptions()
 
     call b:NERDTree.render()

lib/nerdtree/nerdtree.vim

@@ -181,11 +181,6 @@ function! s:NERDTree.PathFilters()
     return s:NERDTree._PathFilters
 endfunction
 
-"FUNCTION: s:NERDTree.previousBuf() {{{1
-function! s:NERDTree.previousBuf()
-    return self._previousBuf
-endfunction
-
 "FUNCTION: s:NERDTree.render() {{{1
 "A convenience function - since this is called often
 function! s:NERDTree.render()