fix unreachable code: revert OperatorNotAllowed(operator);

[felipelincoln]

The following line /src/OperatorFilterer.sol#L56 is unreachable because isOperatorAllowed/2 can only return true or throw the errors AddressFiltered/1 and CodeHashFiltered/2.

To fix it and also make it compatible with documentation:

isOperatorAllowed will return true. Otherwise, isOperatorAllowed will return false

-- /README.md#updatecodehash

This PR refactors isOperatorAllowed/2 removing the reverts and returning false instead.

[felipelincoln]

TODO: fix tests

[haowang1013]

I'm also bothered by the problem "fixed" in this PR but as I understand it the registry contract is not upradable so it's likely that it'll never get fixed on-chain...