Use only mapping for addressOwnershipProofs

[sebastiantf]

Description

Currently both the array addressOwnershipProofs and the mapping _ownershipProofByAddress are being used to track address ownership proofs. But the use of both seems unnecessary, inefficient and error-prone.

Didn't really notice any specific use-case being served by the array.

If there isn't really any use of the array, then I suppose we could remove the array, modify the mapping to store the AddressOwnershipProof struct and only use that. This is probably going to be a lot more efficient and simpler