[release-branch.go1.21] doc: delete go1.21.html copy

When the final Go 1.21.0 release is made, the release notes will lose
the draft status and begin to be visible at https://go.dev/doc/go1.21.
The source go1.21.html file will then move from the main Go repository
to x/website as part of it becoming a finished and eventually frozen
historical release notes page.

The https://tip.golang.org/doc/go1.21 page that currently serves the
current draft always uses the main repository's main branch, so this
copy on the release branch is not used by anything. Its existence on
this branch may attract backports, which are harmless but also not
very useful. Avoid that temptation by deleting it here sooner.

(Since the first RC comes with a mostly complete release notes draft,
maybe in the future we can consider moving this file to x/website
right before a release branch is cut, making the draft available at
https://go.dev/doc/go1.21 sooner so fewer links will need updating.)

For #58645.

Change-Id: I92c0200b748a5f255f9b8113b4952c122631c6d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/511317
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>