Bug 19422: Missing DROP TABLES in kohastructure.sql

Bug 19422: Missing DROP TABLES in kohastructure.sql

Adds DROP TABLE statements to the following tables
    - article_requests
    - biblio_metadata
    - deletedbiblio_metadata

Test plan :
Clean your database by running 'mysql < kohastructure.sql'.
Without patch you'll get error messages.
With patch everything will work fine, no warnings.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>