Remove unnecessary 'menu_name' fallback in get_post_type_labels()

[evansolomon]

get_post_type_labels() sets the fallback for the menu_name label to the name label. Then it calls _get_custom_object_labels(), which does the same thing along with a bunch of similar fallbacks.

This is an artifact of #14832, which lead to r16251, then to r16268, which made the fallback in get_post_type_labels() unnecessary.

We can just remove the statement from get_post_type_labels() and nothing will change (except WordPress will be just a bit simpler).

[duck_]

The line isn't unnecessary. If $object->labels['name'] isn't set at that point then there will be no fallback. For example:

register_post_type( 'example', array( 'public' => true ) );

Now there will be a notice during menu generation and the menu item will have no label.

Maybe it's unlikely to occur, but this fallback code does do something.