tabnr: replace by a new naive function

This function simply returns the current tabnumber followed by the
number of buffers in the current tabpage.

The previous one either was broken long ago or did not work as expected.

fixes #2616

autoload/airline/extensions/tabline/formatters/tabnr.vim

@@ -3,15 +3,8 @@
 
 scriptencoding utf-8
 
-function! airline#extensions#tabline#formatters#tabnr#format(tab_nr_type, nr)
-  " TODO: Is this actually useful? Or should we remove this one?
-  let nr = type(a:nr) == type([]) ? a:nr[0] : a:nr
+function! airline#extensions#tabline#formatters#tabnr#format(nr, buflist)
   let spc=g:airline_symbols.space
-  if a:tab_nr_type == 0 " nr of splits
-    return spc. '%{len(tabpagebuflist('.nr.'))}'
-  elseif a:tab_nr_type == 1 " tab number
-    return spc. nr
-  else "== 2 splits and tab number
-    return spc. nr. '.%{len(tabpagebuflist('.nr.'))}'
-  endif
+
+  return printf("%s %d/%d", spc, a:nr,  len(tabpagebuflist(a:nr)))
 endfunction