Make delete_user action hook helpful

[filosofo]

As I brought up ​here, the 'delete_user' hook is poorly placed, not allowing you to access usermeta data before a user is deleted. The attached patch moves it to a better place.

(For example, you might want to change post_meta data associated with a user's meta data, but by the time of the delete_user callback, all that user's meta data has been deleted.)

Matt suggested ​here that before moving the 'delete_user' hook, one should grep the plugins repository to see if anyone is using it in a plugin. I did, and only one plugin, ​Gengo, does, and it does in an apparently redundant way, as it attempts to delete usermeta files that have already been deleted.

So no plugins would be harmed by moving the 'delete_user' action hook.

(In [4020]) Relocate delete_user action. Props filosofo. fixes #2956