Remove LfsDateTimeTest and dependency on joda-time

LfsDateTimeTest was added in change I49bffffbd when joda-time was
replaced with the Java 8 time API. The intention was to confirm that
timestamps generated with the Java 8 API were the same as the ones
generated with joda-time, to ensure backward compatibility.

It's been more than two years since that change, so we no longer
need the test. Remove it, and thus also remove the dependency on the
joda-time library.

Bug: Issue 12974
Change-Id: Iec71af19534d7df0ce7b768c0ccc772de1cb524f
3 files changed