db: Remove deprecated assert_unicode attribute

Remove deprecated assert_unicode sqlalchemy attribute,
removed in sqlalchemy 0.8.0. The deprecation is described
at:

http://docs.sqlalchemy.org/en/latest/changelog/changelog_08.html#change-ad44af79c886d1bb283042deb64f9cbe

Fixes LP: #1208382

Change-Id: I56ad98d3702f53fe8cfa94093fea89074f7a5e90
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "db: Remove deprecated assert_unicode attribute"