[TASK] Make workspace delete a discard

When deleting a record in workspaces that has been
changed in comparison to live - if it is a new, a moved
or a changed workspace record, those records where subject
to the 'soft-delete' strategy of the core: Soft-delete
enabled tables like pages or tt_content did set deleted=1
for those records instead of fully deleting them from the
table. The idea was to allow 'undelete' via recycler or
history module if those records have been deleted by
accident.

However, the undelete workspace records functionality
never worked and has never been fixed. The oldest open
forge issue on this topic is 11 (!) years old.

The solution is to discard those records instead of
soft-deleting them. This effectively drops soft-delete
for workspace records. It's done by a trivial switch in
the main DataHandler delete method to re-route workspace
delete operation to the recently refactored discard
methods instead.

This change allows us to further refactor the remaining
tasks the DataHandler...