runtime: remove adjustTimers counter

In CL 336432 we changed adjusttimers so that it no longer cleared
timerModifiedEarliest if there were no timersModifiedEarlier timers.
This caused some Google internal tests to time out, presumably due
to the increased contention on timersLock.  We can avoid that by
simply not skipping the loop in adjusttimers, which lets us safely
clear timerModifiedEarliest.  And if we don't skip the loop, then there
isn't much reason to keep the count of timerModifiedEarlier timers at all.
So remove it.

The effect will be that for programs that create some timerModifiedEarlier
timers and then remove them all, the program will do an occasional
additional loop over all the timers.  And, programs that have some
timerModifiedEarlier timers will always loop over all the timers,
without the quicker exit when they have all been seen.  But the loops
should not occur all that often, due to timerModifiedEarliest.

For #47329

Change-Id: I7b244c1244d97b169a3c7fbc8f8a8b115731ddee
Reviewed-on: https://go-review.googlesource.com/c/go/+/337309


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