chore(blobstorage): fix blobstorage-down-migrations

[xiaodino]

Getting an error in blobstorage-down-migrations

2024/05/01 06:08:16 goose run: failed to db-down: ERROR 16669_add_blob_hash_index.sql: failed to run SQL migration: failed to execute SQL query "DROP INDEX blob_hash_index on blob_hashes;": Error 1553 (HY000): Cannot drop index 'blob_hash_index': needed in a foreign key constraint

• Foreign keys in MySQL automatically create an index on the table. So we don't need 16669_add_blob_hash_index.sql
• No need to create index for the primary key
• DROP INDEX blob_hash_index in down migration