fix(similarity): use get_primary_hash in backfill

[JoshFerge]

• previously we were getting a random hash associated with the group.
• now, we simply use event.get_primary_hash()
• removes passing down these hashes through the various functions as its not needed now
• deletes a few tests where we were going through edge cases where we weren't finding a group_id associated with the hash .

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: src/sentry/tasks/backfill_seer_grouping_records.py

Function	Unhandled Issue
backfill_seer_grouping_records	KeyError: 5172984251 sentry.tasks.backfill_seer_g... Event Count: 1

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