runtime: remove scase.releasetime field

This is the gofrontend version of https://golang.org/cl/245122.
Original CL description:

    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 golang/go#40410.

This is being brought over to gofrontend as a step toward upgrading to
Go1.16beta1.

Change-Id: I60688303cdae72f89cfa9777402933ecee2503f7
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/279732
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed