Bug 11945: SQL syntax error in delete_expired_opac_registrations.pl

Bug 11945: SQL syntax error in delete_expired_opac_registrations.pl

This patch corrects a syntax error, adds some comments to the usage
instructions and adds a count of the removed borrowers (with verbose
flag on).

Note that this is a dangerous script. It will delete your patrons
in patron category PatronSelfRegistrationDefaultCategory.
If you do not use this as a temporary category, you should NOT run this
script.

Test plan:
Check PatronSelfRegistrationDefaultCategory.
Check PatronSelfRegistrationExpireTemporaryAccountsDelay.
Based on these two settings, check the number of patrons to be deleted (date
enrolled should be before NOW minus the delay).
Backup your data and run the script.
Check the number of deleted borrowers.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>