This is a revert of CL 24945 and CL 24954, since we no longer need to
support Go 1.5.
No need to explicitly specify which Go version is required. Instead,
rely on the implicit requirement based on the release support policy¹.
This is simpler and consistent with how it was before CL 24954.
¹ https://tip.golang.org/doc/devel/release.html#policy
Change-Id: I3cfef1bef120f702d5859dad10a76803544cc869
Reviewed-on: https://go-review.googlesource.com/c/148569
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
