Remove unused messages 'resetprefs' and 'timezoneoffset'

Follows-up r49790 (155ddf6de4a2).

I would have also deleted 'searchresultshead' had it not been for the fact
that the SemanticDrilldown extension still uses it (albeit incorrectly).
So for now, I am leaving that message in, adding a note that the message
is deprecated.

Change-Id: I4485c6fbe4fc7640c9cb2d765738fbec6bee26ab
Merge "Remove unused messages 'resetprefs' and 'timezoneoffset'"