fix (haskell/gigasecond): Adapt to new test suite
fix (haskell/gigasecond): Adapt to new test suite

New test suite changed types from `Data.Time.Calendar.Day` to
`Data.Time.Clock.UTCTime`.

Change `fromDay` to work with the new types. Remove `gigasecond`, which
was needed for readability in the old version, but is not needed
anymore.

Bugfry/exercises,Bugfry/exercises,Bugfry/exercises,Bugfry/exercises,Bugfry/exercism,Bugfry/exercism,Bugfry/exercism,Bugfry/exercises,Bugfry/exercism,Bugfry/exercism,Bugfry/exercises,Bugfry/exercism