Remove unused lastSeenUpdate from TS snapshot tracker

[tjenkinson]

👋 hey!

Was looking through the code and it doesn't look like this is used anywhere, and I think the existing logic that cleans up based on xmaxSnapshots should cover it?

I think the intention was to use the global_last_seen_lsn added in #3159

See this discussion on discord

[Copilot]

Pull request overview

This PR removes unused snapshot cleanup functionality from the TypeScript client's snapshot tracker that was never integrated into the actual implementation. The cleanup logic based on xmaxSnapshots already provides the necessary functionality.

Changes:

• Removed unused snapshotsByDatabaseLsn tracking map and related logic from SnapshotTracker
• Removed unused lastSeenUpdate method that was never called in the codebase
• Removed the corresponding test for the unused lastSeenUpdate functionality

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/typescript-client/src/snapshot-tracker.ts	Removed unused snapshotsByDatabaseLsn map, removed tracking logic in addSnapshot, and removed the lastSeenUpdate method
packages/typescript-client/test/snapshot-tracker.test.ts	Removed test for the unused lastSeenUpdate method

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