Remove unused internal field _tombstone

[mtrezza]

Summary

The internal field _tombstone is referenced in two places in the codebase but is not used by any feature or logic:

• src/Controllers/DatabaseController.js — listed in specialMasterQueryKeys
• src/Adapters/Storage/Mongo/MongoTransform.js — handled in a case statement

No code in Parse Server reads, writes, or queries this field. It appears to be a leftover from a planned or removed soft-delete feature that was never fully implemented.

Proposal

Remove _tombstone from both locations to reduce dead code.

[parse-github-assistant]

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