[TechDebt]: TWG & Verifier — post Triggerable Withdrawals review cleanup

[eddort]

Summary

Post-review improvements for TriggerableWithdrawalsGateway.sol and ValidatorExitDelayVerifier.sol.

Context

Comments deferred from PR #1382:

• unstructed storage is redundant as it's not a Proxy contract
• _setupRole() is deprecated in favor of {_grantRole}
• check if the method is redundant
• unused error
• fix doc
• fix doc