Bug 14201: Plugin marc21_leader_book is useless

Bug 14201: Plugin marc21_leader_book is useless

A closer look at this plugin revealed the following:
[1] The leader_book builder redirects to leader.pl; the launcher code with
    leader_book.tt is dead.
[2] A diff of the two templates showed that most changes are minimal and
    irrelevant. Some updates to leader have not been incorporated into
    the unused leader_book.
[3] Leader_book does not use Focus while leader does.

This patch removes all traces of leader_book :)
It includes a tiny db revision for those brave people using it. The only
behavior change for them is the focus change.
Another small change: we do not need Search.pm in marc21_leader.

Test plan:
[1] Run the db revision. If you had some field attached to leader_book,
    it should be gone.
[2] Check if the leader plugin still works as expected.
[3] Run the FrameworkPlugin unit test. Ignore warnings from field_007.
[4] Git grep on marc21_leader_book. You will only find references to
    PO files in misc/translator.

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>