fix: remove tombstone from validator_status (already exists in validator_singing_info)

[huichiaotsou]

Description

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. #411

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I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
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[MonikaCat]

tested, LGTM

[RiccardoM]

Can you just add a CHANGELOG entry please?