API: don't directly call update_screen() in API functions

There is no need to call update_screen() directly in an API function,
mode input processing invokes update_screen() as needed. And if the API
call is done in a context where redraw is disabled, then redraw is
disabled for a reason. A lot of API functions are of equal semantical
strength (nvim_call_function and nvim_execute_lua can also do whatever,
nvim_command is not special), this inconsistency has no purpose.

src/nvim/api/vim.c

@@ -75,7 +75,6 @@ void nvim_command(String command, Error *err)
 {
   try_start();
   do_cmdline_cmd(command.data);
-  update_screen(VALID);
   try_end(err);
 }
 

src/nvim/api/window.c

@@ -135,7 +135,7 @@ void nvim_win_set_cursor(Window window, ArrayOf(Integer, 2) pos, Error *err)
   // make sure cursor is in visible range even if win != curwin
   update_topline_win(win);
 
-  update_screen(VALID);
+  redraw_win_later(win, VALID);
 }
 
 /// Gets the window height