fix: soft delete empty repository records (batch 3)

This is part of step 3 of 4 as described in #570 (comment 818150257).

We're doing a batch update as first described in #570 (comment 838050935). This MR is for batch 3, which is split into four migrations, each one with a single update statement targetting 10k records.

Based on the production data after deploying !895 (merged):

gitlabhq_registry=> select * from schema_migrations order by id desc limit 3;
                          id                           |          applied_at
-------------------------------------------------------+-------------------------------
 20220216124355_soft_delete_emtpy_repositories_batch_2 | 2022-02-16 16:23:10.932079+00
 20220216122704_soft_delete_emtpy_repositories_batch_2 | 2022-02-16 16:23:10.141892+00
 20220215130020_soft_delete_emtpy_repositories_batch_1 | 2022-02-16 11:35:43.58211+00
(3 rows)

We can now be confident that it takes ~790ms for each 10k update. So we're increasing the number of updates per deployment.

Related to #570 (closed)