Bug 14201: We can do without plugin marc21_leader_computerfile

Bug 14201: We can do without plugin marc21_leader_computerfile

A closer look at this plugin revealed the same things as the previous
patch for leader_book.
[1] Template leader_computerfile is not used.
[2] The unused template is not up-to-date either.
[3] Leader_computerfile does not use Focus while leader does.

This patch removes all traces of leader_computerfile.
It includes a tiny db revision for those brave people using it. The only
behavior change for them is the focus change.

Test plan:
[1] Run the db revision.
[2] Check if the leader plugin still works as expected.
[3] Git grep on marc21_leader_computerfile. Ignore PO files.

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>