doc/next: delete

The release note fragments have been merged and added
as _content/doc/go1.27.md in x/website in CL 784681.

For #78779.

Change-Id: I2333d1a9f55158d4e16351a5c53252bf76c3288e
Reviewed-on: https://go-review.googlesource.com/c/go/+/784700
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>