removing the list container from the custom menu output

The container argument controls what is one level higher – around <ul>.

What you need is items_wrap argument that controls the list wrap and defaults to <ul id="%1$s" class="%2$s">%3$s</ul>.

Removing the ul wrap in Codex gives following example:

wp_nav_menu( array( 'items_wrap' => '%3$s' ) );