remove the NERDChristmasTree option

If users want to customise the tree colours, this can be done trivially
in their vimrc. e.g.

    hi link NERDTreeFile error

doc/NERD_tree.txt

@@ -609,12 +609,10 @@ NERD tree. These options should be set in your vimrc.
 
 |'loaded_nerd_tree'|            Turns off the script.
 
-|'NERDChristmasTree'|           Tells the NERD tree to make itself colourful
-                                and pretty.
-
 |'NERDTreeAutoCenter'|          Controls whether the NERD tree window centers
                                 when the cursor moves within a specified
                                 distance to the top/bottom of the window.
+
 |'NERDTreeAutoCenterThreshold'| Controls the sensitivity of autocentering.
 
 |'NERDTreeCaseSensitiveSort'|   Tells the NERD tree whether to be case
@@ -695,15 +693,6 @@ If this plugin is making you feel homicidal, it may be a good idea to turn it
 off with this line in your vimrc: >
     let loaded_nerd_tree=1
 <
-------------------------------------------------------------------------------
-                                                         *'NERDChristmasTree'*
-Values: 0 or 1.
-Default: 1.
-
-If this option is set to 1 then some extra syntax highlighting elements are
-added to the nerd tree to make it more colourful.
-
-Set it to 0 for a more vanilla looking tree.
 
 ------------------------------------------------------------------------------
                                                         *'NERDTreeAutoCenter'*

plugin/NERD_tree.vim

@@ -45,7 +45,6 @@ function! s:initVariable(var, value)
 endfunction
 
 "SECTION: Init variable calls and other random constants {{{2
-call s:initVariable("g:NERDChristmasTree", 1)
 call s:initVariable("g:NERDTreeAutoCenter", 1)
 call s:initVariable("g:NERDTreeAutoCenterThreshold", 3)
 call s:initVariable("g:NERDTreeCaseSensitiveSort", 0)

syntax/nerdtree.vim

@@ -63,15 +63,10 @@ syn match NERDTreeBookmarksHeader #^>-\+Bookmarks-\+$# contains=NERDTreeBookmark
 syn match NERDTreeBookmarkName #^>.\{-} #he=e-1 contains=NERDTreeBookmarksLeader
 syn match NERDTreeBookmark #^>.*$# contains=NERDTreeBookmarksLeader,NERDTreeBookmarkName,NERDTreeBookmarksHeader
 
-if exists("g:NERDChristmasTree") && g:NERDChristmasTree
 hi def link NERDTreePart Special
 hi def link NERDTreePartFile Type
 hi def link NERDTreeExecFile Title
 hi def link NERDTreeDirSlash Identifier
-else
-    hi def link NERDTreePart Normal
-    hi def link NERDTreePartFile Normal
-endif
 
 hi def link NERDTreeBookmarksHeader statement
 hi def link NERDTreeBookmarksLeader ignore