Bug 22580: Remove deprecated delete_expired_opac_registrations.pl cronjob

Bug 22580: Remove deprecated delete_expired_opac_registrations.pl cronjob

Removes the deprecated script and changes the hint on the
PatronSelfRegistrationExpireTemporaryAccountsDelay sytem preference.

To test:
- Make sure the script misc/cronjobs/delete_expired_opac_registrations.pl
  was deleted
- Make sure the hint on the PatronSelfRegistrationExpireTemporaryAccountsDelay
  system preference is correct
- Make sure there are no other reference to the removed script

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>