Remove DateTime implementation, remove Delegation

DateTime inherits from Date, so we don't need to implement in both. And
the DateTime implementation was wrong, anyway.

Using SimpleDelegation for Gigaseconds was a bad idea as delegation
wasn't needed. Imagine this code:

```ruby
d = Gigasecond.new(Date.today)
irb(main):011:0> d.date
irb(main):012:0> d.to_time
```

`date` returns a date 32 years in the future, while `to_time` returns
today. Bad idea.