No point doing this check all the time.

Fixes #15621

Change-Id: I1966c061986fe98fe9ebe146d6b9738c13cef724
Reviewed-on: https://go-review.googlesource.com/30670Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>