[Refactor] convert postgres delete implementation to parameterized query

[kurt-r2c]

Pull Request (PR) Checklist

Short Description:

Refactor the Postgres datastore delete implementation to use parameterized queries.

Issue(s) Linked: N/A

Branch:

Branch in fork: https://github.com/kurt-r2c/chatgpt-retrieval-plugin/tree/kb/postgres-delete-parameterized-query

Code Changes & Tests:

Minimal changes - existing Postgres test suite passed.