runtime, cmd/compile: prune unused _defer fields

runtime, cmd/compile: prune unused _defer fields

These fields are no longer needed since go.dev/cl/513837.

Change-Id: I980fc9db998c293e930094bbb87e8c8f1654e39c
Reviewed-on: https://go-review.googlesource.com/c/go/+/516198
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>