Code Quality: Remove redundant variable assignment in `wp_list_authors()`.

There is no need to initialize `$post_counts` as an empty array, since it will be overwritten directly by the filter value in the next line.

Follow-up to [54262].

Props Soean.
See #64238.

git-svn-id: https://develop.svn.wordpress.org/trunk@62091 602fd350-edb4-49c9-b593-d223f7449a82