chore(pallet-nfts): DelegateApprovalConflicts

[chungquantin]

Link to the discussion: #387 (comment)

Note: Implementation was initially inspired from AZ: https://github.com/Cardinal-Cryptography/PSP34/blob/main/data.rs#L120

Description

The current implementation throws an error DelegateApprovalConflicts when attempting to revoke approval for a single token if the operator already has approval for all tokens. This issue proposes revisiting whether this error is necessary, as its removal does not appear to impact other methods, such as transfer, or introduce any security concerns.

This might be an edge case, but its worth considering why someone might have these approvals. If it is purely for the sake of listing it on a market place, say some marketplace contract, then what might the expected usage be.