Remove g:startify_session_detection

This was always enabled by default and no one seems to disable it.

autoload/startify.vim

@@ -68,7 +68,7 @@ function! startify#insane_in_the_membrane(callingbuffer) abort
 
   let cnt = 0
 
-  if get(g:, 'startify_session_detection', 1) && filereadable('Session.vim')
+  if filereadable('Session.vim')
     call append('$', ['   [0]  '. getcwd() . s:sep .'Session.vim', ''])
     execute 'nnoremap <buffer> 0 :call startify#session_delete_buffers() <bar> source Session.vim<cr>'
     let cnt = 1

doc/startify.txt

@@ -90,7 +90,6 @@ default values.
     |g:startify_list_order|
     |g:startify_bookmarks|
     |g:startify_files_number|
-    |g:startify_session_detection|
     |g:startify_session_autoload|
     |g:startify_session_persistence|
     |g:startify_session_delete_buffers|
@@ -188,14 +187,6 @@ Example:
 <
 The number of files to list.
 
-------------------------------------------------------------------------------
-                                                  *g:startify_session_detection*
->
-    let g:startify_session_detection = 1
-<
-When the file Session.vim is found in the current directory, it will be shown
-at the top of all lists as entry [0].
-
 ------------------------------------------------------------------------------
                                                    *g:startify_session_autoload*
 >