runtime: update _defer comment to not mention freedefer

runtime: update _defer comment to not mention freedefer

CL 339669 changed freedefer to not mention every field of _defer,
so no need to call it out in the _defer comment.

Change-Id: Id8b67ba2298685f609bf901b5948fd30666bd6e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/382251
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>