Relax implicit account requirements

[tadzik]

Description of change

Following the changes in iotaledger/iota.go#604, we relax the requirements for what we treat as an implicit account creation.

The restrictions in there wrt the allowed featureset is something we'll need to add later, once we introduce the creation of implicit accounts.

Type of change

• Breaking change (is it?)

How the change has been tested

The tests still pass, but they don't to exercise this code path at all. Is it the right time to introduce them?

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes