map_meta_cap() doesn't require $post_author_data

[duck_]

map_meta_cap() sets up $post_author_data to check if a user is editing/deleting/reading their own post. However, the only piece of data required is the ID. To get this data we already have $post_author_id from $post->post_author or $user_id.

Related: [21563] for #21120 did some optimisations to avoid get_userdata()

[nacin]

Passes all existing tests.

[nacin]

In 25177:

Don't set up $post_author_data in map_meta_cap() as we don't need it.

props duck_.
fixes #25177.