Bug 11391: drop default value on suggestions.suggesteddate column

Bug 11391: drop default value on suggestions.suggesteddate column

The 'default 0' clause got translated as an invalid constant
default of '0000-00-00' when DBIx::Schema is used to deploy
the suggestions table into a Pg database.  This patch drops
the default.

To test:

[1] Apply the patch and run the SQL specified in the database
    updated.
[2] Verify that the suggestions table no longer has an
    explicit default value for the suggesteddate column.
[3] Verify that prove -v t/db_dependent/Suggestions.t
    passes.
[4] Verify that installer/data/mysql/kohastructure.sql runs
    cleanly in an empty database.
[5] Verify that there are no visible regressions of the
    purchase suggestions functionality.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Having a default of 0 on a date seems like a mad thing to do anyway,
so good to get rid of it

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>