Bug 16565: Drop additional_fields and additional_field_values before creating them

Bug 16565: Drop additional_fields and additional_field_values before creating them

These 2 tables should be dropped before trying to create them

Test plan:
From the SQL CLI, source the kohastructure.sql file
source it again
=> Without this patch you get 2 warnings
ERROR 1050 (42S01) at line 3580 in file:
'installer/data/mysql/kohastructure.sql': Table
'additional_fields' already exi
sts
ERROR 1050 (42S01) at line 3596 in file:
'installer/data/mysql/kohastructure.sql': Table
'additional_field_values' alrea
dy exists

=> With this patch, you won't get them

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>