Remove the obsolete `GetRandomStringCombination` method from the `RandomizationHelper` class

[TonyTroeff]

Every obsolete method should be removed upon the next major release. The GetRandomStringCombination(int, IReadOnlyList<char>) method is one of them.

[postmeback]

I am interested. Shall I take it?

[TonyTroeff]

Sure, you can work on this issue. However, the required changes will not be merged until it is required to release a new major version of TryAtSoftware.Randomizer.

[postmeback]

Ok.