Bug 22128: Use DROP USER IF EXISTS instead of GRANT USAGE before DROP USAGE

Bug 22128: Use DROP USER IF EXISTS instead of GRANT USAGE before DROP USAGE

MySQL 5.7 is old, and the latest versions fail on "GRANT USAGE" if the
user does not exist. "DROP USER IF EXISTS" has been with us since 5.7,
so all users should have it by now. This patch fixes the issue.

To test:

Try to koha_remove a site, and if it works with no errors, all good!

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>