show_date_relative(): drop unused "tz" parameter

show_date_relative(): drop unused "tz" parameter

The timestamp we receive is in epoch time, so there's no need for a
timezone parameter to interpret it. The matching show_date() uses "tz"
to show dates in author local time, but relative dates show only the
absolute time difference. The author's location is irrelevant, barring
relativistic effects from using Git close to the speed of light.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>