Remove antiquated a map

Resolves: #2380

Author: tpope

autoload/fugitive.vim

@@ -2697,7 +2697,7 @@ function! s:MapStatus() abort
   call s:MapMotion('gP', "exe <SID>StageJump(v:count, 'Unpulled')")
   call s:MapMotion('gr', "exe <SID>StageJump(v:count, 'Rebasing')")
   call s:Map('n', 'C', ":echoerr 'fugitive: C has been removed in favor of cc'<CR>", '<silent><unique>')
-  call s:Map('n', 'a', ":<C-U>execute <SID>Do('Toggle',0)<CR>", '<silent>')
+  call s:Map('n', 'a', ":echoerr 'fugitive: a has been removed in favor of s'<CR>", '<silent><unique>')
   call s:Map('n', 'i', ":<C-U>execute <SID>NextExpandedHunk(v:count1)<CR>", '<silent>')
   call s:Map('n', "=", ":<C-U>execute <SID>StageInline('toggle',line('.'),v:count)<CR>", '<silent>')
   call s:Map('n', "<", ":<C-U>execute <SID>StageInline('hide',  line('.'),v:count)<CR>", '<silent>')