Remove getDayDifference and its test

* The method is not in use after merging contacts and contactsCommon.
* And its test is flaky.

Bug 27346234

Change-Id: If7a9f18bbc57b76a2fc60a8ecc0d0e636547867f
(cherry picked from commit b6a0256a9f9f67964f2129049179f30e90abd5ea)
2 files changed