Sitemaps: Rename `wp_get_sitemaps()` to `wp_get_sitemaps_providers()`

Following [48536], rename the function to match the rest of the sitemaps logic.

Also eliminates some dead code after [48523].

Props pbiron.
See #50724. See #50643.
Built from https://develop.svn.wordpress.org/trunk@48540


git-svn-id: https://core.svn.wordpress.org/trunk@48302 1a063a9b-81f0-0310-95a4-ce76da25c4cd