Remove dependency on unic-ucd-common by using std::char::is_control

[mmalecot]

Checking whether a character is a control character is already present in the std, so no external dependency is required and the unic feature no longer needs to exist (or at least is built-in).

[Keats]

Nice thanks, are you using that validator? That's the one I didn't know whether it was used by anyone at all

[mmalecot]

Yes, I use it to ensure that a string contains only single-line text for example.