ui: Remove redundant ui.h includes

Also move read_error_exit to os/input.c

src/nvim/ui.c

@@ -93,17 +93,6 @@ int ui_get_shellsize(void)
   return retval;
 }
 
-/*
- * Exit because of an input read error.
- */
-void read_error_exit(void)
-{
-  if (silent_mode)      /* Normal way to exit for "ex -s" */
-    getout(0);
-  STRCPY(IObuff, _("Vim: Error reading input, exiting...\n"));
-  preserve_exit();
-}
-
 /*
  * May update the shape of the cursor.
  */