fix: remove tombstone from validator_status (already exists in `val…

…idator_singing_info`) (forbole#443)

Closes: #XXXX

Removing `tombstone status` from `validator_status` because:
- it makes the code cleaner as `tombstone status` is already stored in `validator_singing_info`
- it saves grpc calls for each validator at each block
- the front-end `graphQL` does not query it from `validator_status` table anyways

c.f. forbole#411

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