Clean TicketController Class

• refactor name of private void checkStudentCustomerIsNotTooOld(TicketValidationRequest body) function because the name no longer describes what it does
• look at this exception - can we remove it because all datetimes are already checked when this gets executed?
• this gets never thrown because checkValidityOfDiscountCard() already throws the exception (combined with the fact above) - please make it more concise for example by making the checkValidityOfDiscountCard() function never throw but always return