runtime: remove GoSched/GoStart trace events around GC

runtime: remove GoSched/GoStart trace events around GC

These were appropriate for STW GC, since it interrupted the allocating
Goroutine, but don't apply to concurrent GC, which runs on its own
Goroutine. Forced GC is still STW, but it makes sense to attribute the
GC to the goroutine that called runtime.GC().

Change-Id: If12418ca66dc7e53b8b16025af4e03adb5d9577e
Reviewed-on: https://go-review.googlesource.com/8715
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>