Remove Settings.System.DATE_FORMAT

Remove references to Settings.System.DATE_FORMAT and mark it as clearly
deprecated.

It was probably made obsolete some time before kitkat. There were still
some references in view code up to lollipop (removed in change
Ib77a8e7727d027cae39d5e6f431cac1d1ff8a121). During marshmallow / nougat
there were no references, then references were added in backup code in
oreo. This change is intended to make it more obvious that this setting
is obsolete.

Bug: 185884644
Test: build only
Change-Id: I12441541bc3ca0e012cee64d4c784a0ce8233715