16349: Skip converting DB timestamps to & from localtime.

16349: Skip converting DB timestamps to & from localtime.

The previous code already returned the correct time. This change
merely avoids converting the time to the local time zone in
PostgreSQL, and back to UTC again in Rails.

This might also avoid rare problems in unusual edge cases where Rails
and PostgreSQL session use different time zones, like a configuration
mishap or a race during a daylight savings time transition.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@tomclegg.ca>