tests: timer_api: Fix duplicate timestamp update.

Remove duplicate "tdata.timestamp" update in duration_expire; this
value is already updated by k_uptime_delta.

Besides simply removing duplicate value update, this commit also
addresses the intermittent assertion failure that is caused by
updating "tdata.timestamp" at a later time than the actual execution
of the k_uptime_delta function.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>