[WP 4.6] Use `get_current_network_id()` instead of `$wpdb->siteid`

[JDGrimes]

https://core.trac.wordpress.org/ticket/41507#comment:12:

Multisite: Remove references to $wpdb->siteid and use get_current_network_id() instead.

[kyam-mayden]

Looking at this if no-one else is?