Remove support for "AddValidatorTx" transaction type

[JacobEverly]

Discontinue the 'AddValidatorTx' transaction, originally used for validator additions to the main network. Since the introduction of 'AddPermissionlessValidatorTx' in v1.9.0, two versions prior to our current v1.11.0, 'AddValidatorTx' has been superseded. The newer transaction type necessitates validators to provide Proof of Possession for their BLS key. This key, stored on the validator node, when registered on-chain, allows for upgrades to be made that significantly improves the scalability of the P-Chain, unblocking additional features for subnets.