sort: drop implementation for Go <1.21

Now that Go 1.22.6 is the minimum bootstrap toolchain (cf. CL 606156),
the fallback implementation for Go versions <1.21 can be dropped.

For #61180
For #64751

Change-Id: Idfeca0a6e9f490e1ab0f308ead372612402923ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/607315


Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Commit-Queue: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>