Customizer: Remove a line of commented-out code in `WP_Customize_Nav_Menus::customize_register()`.

Retains a more explicit future-compat version of the todo notation that used to accompany the commented-out code.

Props harsh175, dlh.
Fixes #44633.

Built from https://develop.svn.wordpress.org/trunk@44328


git-svn-id: https://core.svn.wordpress.org/trunk@44158 1a063a9b-81f0-0310-95a4-ce76da25c4cd