[TechDebt]: NodeOperatorRegistry.sol — post Triggerable Withdrawals review cleanup

[eddort]

Summary

Post-review improvements for NodeOperatorRegistry.sol.

Context

Comments deferred from PR #1382:

• remove stuck operator mention
• preserve a piece of doc
• comment about penalties might be not applicable
• unused:
  MAX_STUCK_PENALTY_DELAY
  stuckPenaltyStats ⇒ stuckPenaltyStats_deprecated
• line below refers to stuck
• line 532: stuck
• cleanup spec
• unused check
• rename exited -> late
• Low - view method: Possible, subtraction underflow
• Low - checked in validator verifier: Possible, subtraction underflow

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Security improvement:
#1510