doc/next: delete

The release note fragments have been merged and added
as _content/doc/go1.23.md in x/website in CL 594035.

For #65614.

Change-Id: I7c8c718bc065024b6bdca93ce9bcc3c5d8b2dbc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/594036
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: gopherbot