Remove validates_timeliness

This library has been abandoned, has been producing deprecation warnings and is only used to validate a single field so remove it and perform the validation ourselves.

This is of arguable value but the deprecation warnings were annoying me. Also, the validator is only tested by the model specs.

Any opinions?
Looks cleaner, though the last conditional could be a bit more concise.

`record.errors.add(attribute, 'must be in the past') unless value.past?`