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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go