[ticket/11862] Correct var names in user_delete() events due to prune-users

The prune-users feature modified the names of the used variables in this
function. However, the variable names for the two events in this function
were not changed to reflect that. This patch will take care of it.

PHPBB3-11862
Merge remote-tracking branch 'Marc/ticket/11862' into develop

* Marc/ticket/11862:
  [ticket/11862] Correct var names in user_delete() events due to prune-users