time, runtime: don't set timer when = 0

timer when == 0, in the context of timer0When and timerModifiedEarliest,
is a sentinel value meaning there are no timers on the heap.
TestCheckRuntimeTimerOverflow reaching into the runtime to set a timer
to when = 0 when it is otherwise not possible breaks this invariant.

After golang.org/cl/258303, we will no longer detect and run this timer,
thus blocking any other timers lower on the heap from running. This
manifests as random timers failing to fire in other tests.

The need to set this overflowed timer to when = 0 is gone with the old
timer proc implementation, so we can simply remove it.

Fixes #42424

Change-Id: Iea32100136ad8ec1bedfa77b1e7d9ed868812838
Reviewed-on: https://go-review.googlesource.com/c/go/+/274632


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Pratt <mpratt@google.com>