startup: remove workaround for skipping redraw

commit 7d082c03b2 added a workaround to skip
a few redraws on startup so that the intro message is not shown anymore.

However might cause problems, that some highlighting groups are not
correctly set. Also recent Vims (8.0.1290 and so on) do not seem to
suffer from the problem that an too early initializiation of airline
causes a redraw which skips the intro message.

Therefore, revert that commit.

Fixes #1585

autoload/airline.vim

@@ -143,11 +143,6 @@ function! airline#statusline(winnr)
 endfunction
 
 function! airline#check_mode(winnr)
-  if !exists("s:airline_run")
-    let s:airline_run = 0
-  endif
-  let s:airline_run += 1
-
   let context = s:contexts[a:winnr]
 
   if get(w:, 'airline_active', 1)
@@ -190,13 +185,6 @@ function! airline#check_mode(winnr)
   endif
 
   let mode_string = join(l:mode)
-  if s:airline_run < 3
-    " skip this round.
-    " When this function is run too early after startup,
-    " it forces a redraw by vim which will remove the intro screen.
-    let w:airline_lastmode = mode_string
-    return ''
-  endif
   if get(w:, 'airline_lastmode', '') != mode_string
     call airline#highlighter#highlight_modified_inactive(context.bufnr)
     call airline#highlighter#highlight(l:mode, context.bufnr)