Bug 29631: Prevent uniq_lang creation to fail

Bug 29631: Prevent uniq_lang creation to fail

We got 2 reports of this problem in the last 24h, uniq_lang unique key is failing to be created because several rows with the same (subtag, type) exist in DB.

I have no idea how this is possible, but apparently it is.

Test plan:
Checkout a commit before 21.06.00.012
reset_all
Create duplicate in language_subtag_registry
> insert into language_subtag_registry(subtag, type, description) values('IN', 'region', 'India');
Checkout master+this patch, updatedatabase
=> Only 1 IN-region exists in the DB, the last one.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>