Remove `externalDisputeID` as pre-dispute evidence submission is not needed anymore

[jaybuidl]

Rationale

The externalDisputeID (aka evidenceGroupID in v1 ERC-1497) has been introduced to allow pre-dispute evidence submission.

Following the conclusion of #2160, the protocol only supports evidence submission by the user (not the Arbitrable contract) for an already existing dispute.

Impact

IEvidence.Evidence

• Replace externalDisputeID by _arbitratorDisputeID

IArbitrable.DisputeRequest

• Remove externalDisputeID.
• _arbitratorDisputeID is already present there.