Bug 30939: Fix use statement for DelAuthority

Bug 30939: Fix use statement for DelAuthority

Without this patch, the script won't delete any
unused authorities, but gives an error instead
and dies:

Undefined subroutine &main::DelAuthority called at ./misc/migration_tools/remove_unused_authorities.pl line 98.

To test:
- Run from koha-shell:
  ./misc/migration_tools/remove_unused_authorities.pl -t
- Verify several authorities are reported as unused
- ./misc/migration_tools/remove_unused_authorities.pl -c
- Verify the error message is shown when the first unused
  authority is found and the script stops
- Apply patch and rerun:
  ./misc/migration_tools/remove_unused_authorities.pl -t
- Verify the error is gone, the script finishes and auhorities
  are deleted

https://bugs.koha-community.org/show_bug.cgi?id=30936

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>