Mollie supports Customer removal

[NathanMolTechno]

Mollie currently supports customer removal. I guess, at the time that this API was written, it was not supported yet? It really is a super small change (Customers API should also implement Delete)

[tubbynl]

i'm not sure how @stil4m is in it's time, but i'm guessing it always makes sense to provide a pull request for the feature that will save him time