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
Change-Id: Ic9e903eceab3c096426f313bf4ef3a38644774e1