New installations haven't had this unique key since 2014
(d9e24d9703) and only old
installations passing through 3.1.0~a would have had it
added.
Given that the unique key constraint was never dropped during
an AlternC upgrade, I think it should just be dropped here.
Re-creating is problematic when merged with other recent changes
because the length of both the key and column size are longer
than what MySQL/MariaDB support.
Kienan Stewart
Guillaume Est Une Palourde