runtime: remove scase.releasetime field

selectgo will report at most one block event, so there's no need to
keep a releasetime for every select case. It suffices to simply track
the releasetime of the case responsible for the wakeup.

Updates #40410.

Change-Id: I72679cd43dde80d7e6dbab21a78952a4372d1e79
Reviewed-on: https://go-review.googlesource.com/c/go/+/245122


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>