Simplify GregorianCalendar methods

[RogerRiggs]

Simplify the additional methods in java.util.GregorianCalendar and rely on the Theeten types with "from" methods to create ThreeTen instances. GregorianCalendar implements the DateTime interface which provides the necessary fields and types.

Remove:
LocalDate toLocalDate()
LocalDateTime toLocalDateTime()
LocalTime toLocalTime()
OffsetDate toOffsetDate()
OffsetDateTime toOffsetDateTime()
OffsetTime toOffsetTime()
ZonedDateTime toZonedDateTime()

[jodastephen]

LocalDate.from(gregCal) is an acceptable alternative.

I would suggest keeping the toInstant() method on j.u.Date and Calendar however, as that is a useful conversion.

[RogerRiggs]

Changed the milestone; the integration of java.util.Calendar/date will be done after the integration into the JDK.