Remove vestigial "Choose date format" option.

Almost all of our date formatting goes via icu these days, so this is
a no-op almost everywhere. (And should be a no-op everywhere soon.)
There's no obvious way to fix it, so remove it. UI guidelines say we
should use spelled out month names anyway.

Bug: 18322220

(cherry picked from commit 30cc3086)

Change-Id: I2a84d69a1f21f71d8bce5b69202b07f1b422aa57