Remove DateTypeAdapter to avoid code duplication

[Marcono1234]

Purpose

Remove the internal DateTypeAdapter class to avoid code duplication

Description

The same functionality exists in DefaultDateTypeAdapter (with some slight overhead though), and there is currently code duplication between these classes.

The code in DefaultDateTypeAdapter and the way it is used now as replacement should behave the same way DateTypeAdapter did, but feel free to double-check in case I overlooked something.

Checklist

• New code follows the Google Java Style Guide
  This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.
  Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[eamonnmcmanus]

This is great, thanks! I think I've said before that the way it handles Date is probably Gson's biggest mistake. We can't easily fix that, but we can at least make it easier to improve the situation.

The changes look good to me. I'm running this past Google's tests in case that turns anything up.

[eamonnmcmanus]

Google's internal tests all passed.