Remove g:startify_empty_buffer_key

This is an unnecessary option. It can be done like this:

    autocmd User Startified nnoremap <buffer><silent>X :enew<cr>

autoload/startify.vim

@@ -143,10 +143,6 @@ function! startify#insane_in_the_membrane(callingbuffer) abort
   nnoremap <buffer>         <2-LeftMouse> :execute 'normal' matchstr(getline('.'), '\w\+')<cr>
   nnoremap <buffer><silent> q             :call <sid>close()<cr>
 
-  if exists('g:startify_empty_buffer_key')
-    execute 'nnoremap <buffer><silent> '. g:startify_empty_buffer_key .' :enew<cr>'
-  endif
-
   autocmd startify CursorMoved <buffer> call s:set_cursor()
   if s:restore_position
     autocmd startify BufReadPost * call s:restore_position()

doc/startify.txt

@@ -101,7 +101,6 @@ default values.
     |g:startify_custom_header|
     |g:startify_custom_footer|
     |g:startify_restore_position|
-    |g:startify_empty_buffer_key|
     |g:startify_enable_special|
     |g:startify_session_savevars|
     |g:startify_session_savecmds|
@@ -279,15 +278,6 @@ Example:
 >
     let g:startify_skiplist_server = [ 'GVIM' ]
 <
-------------------------------------------------------------------------------
-                                                   *g:startify_empty_buffer_key*
->
-    let g:startify_empty_buffer_key = 'o'
-<
-This creates an additional, hidden mapping for the empty buffer.
-
-Default: does not exist
-
 ------------------------------------------------------------------------------
                                                      *g:startify_enable_special*
 >