Your remove_action has to have a priority matching the priority used in add_action.
Important: To remove a hook, the $function_to_remove and $priority
arguments must match when the hook was added. This goes for both
filters and actions. No warning will be given on removal failure.
In you case, it looks like remove_action('wp_head', 'theme_metas'); should work, but you may be having trouble because of how and when your custom.php file loads.