Bug 21865: add confirm to remove_unused_authorities.pl script

Bug 21865: add confirm to remove_unused_authorities.pl script

Like in most scripts in misc, add confirm argument
to ensure script is not run without knowing what it does.

Test plan:
1) Run misc/migration_tools/remove_unused_authorities.pl -h
2) You see help line for confirm
3) Run misc/migration_tools/remove_unused_authorities.pl
4) You see help and script does nothing
5) Run misc/migration_tools/remove_unused_authorities.pl -c
6) Script runs like wanted

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>