Fix: Remove function implementations to force return type to be checked.

[stewartHutchins]

When completing the "generic" challengers the return type is not enforced. e.g.

def add[T](a: T, b: T) -> T:
    return a

def add[T](a: T, b: T):
    return a

both the above functions pass the first "generic" challenge

By removing the function implementation (return a) and replacing it with ..., the 2nd function in the above code snippet becomes an invalid.