doc/next: delete

The release note fragments have been merged and added
as _content/doc/go1.26.md in x/website in CL 726360.

For #75005.

Change-Id: I13533b8a6aac470afe066ddf420028c0360a5bb1
Reviewed-on: https://go-review.googlesource.com/c/go/+/726341
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>