Remove index_ci_builds_on_token_encrypted from ci_builds

For partitioning ci_builds we had to include the partitioning key on all unique indexes, so all the queries supported by index_ci_builds_on_token_encrypted are also supported by unique_ci_builds_token_encrypted_and_partition_id, making index_ci_builds_on_token_encrypted redundant:

    "index_ci_builds_on_token_encrypted" UNIQUE, btree (token_encrypted) WHERE token_encrypted IS NOT NULL
    "unique_ci_builds_token_encrypted_and_partition_id" UNIQUE, btree (token_encrypted, partition_id) WHERE token_encrypted IS NOT NULL

Proposal

1. Disable index_ci_builds_on_token_encrypted via a change request. We want to do this step to lower the risks because we are working with a large table and recreating the index would be impossible during the week since it would take more than 12 hours. unique_ci_builds_token_encrypted_and_partition_id can support all the queries that the old one does, so there will be no surprises here.
2. Write an async migration to drop the index on .com.
3. Write a sync migration to drop the index everywhere else.
4. Celebrate that ci_builds size was reduced by 252 GB