Ripping out no-longer-needed code. Again.

RSpec: 714 examples, 0 failures
RuboCop: 178 files inspected, no offenses detected
More cleanup.

`UserRepository#find_by_name` was really used only in one place, that
itself was part of a larger class that wasn't being used. Whatever I'd
intended `UserDataValidator` for, I'm not using it now and I've
forgotten the original reason. So out it goes.

RSpec: 674 examples, 0 failures
RuboCop: 170 files inspected, no offenses detected