The preferred method of creating a settings page is to use the WordPress Settings API.
From your code example, it looks like it’s too late to remove the admin_notices hook inside your callback in the admin_menu hook.
You should try to move the logic from the customfunc() callback into another callback activated from some earlier hook, for example admin_init.
Additionally:
You are also calling the new admin menu page settings, but the user might confuse it with the existing Settings page.
The User Levels you have in one of the add_menu_page() input parameters, are deprecated, use instead some of the Capabilites mentioned here.
So instead use for example:
add_menu_page( 'My settings', 'My settings', 'manage_options', 'custom', 'customfunc' );