Revert "Cells: Add composite index index_ssh_signatures_on_project_id_and_commit_sha for ssh_signatures table"

Purpose of revert

Original MR: Cells: Add composite index `index_ssh_signature... (!214739 - merged)

#inc-6216-500-errors-from-ssh-signature-endpoints

ActiveRecord::RecordNotUnique
PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_ssh_signatures_on_commit_sha" (ActiveRecord::RecordNotUnique)
DETAIL:  Key (commit_sha)=(\x112345) already exists.

Checklist

• Create an issue to reinstate the merge request and assign it to the author of the reverted merge request.
• If the revert is to resolve a broken 'master' incident, please read through the Responsibilities of the Broken master resolution DRI.
• If the revert involves a database migration, please read through Deleting existing migrations.
• Add the appropriate labels before the MR is created. We can skip CI/CD jobs only if the labels are added before the CI/CD pipeline is created.

Milestone info

• I am reverting something in the current milestone. No changelog is needed, and I've added a ~"regression:*" label.
• I am reverting something in a different milestone. A changelog is needed, and I've removed the ~"regression:*" label.

Related issues and merge requests