constraints: remove package

It has moved to golang.org/x/exp/constraints. Perhaps it will move
back to the standard library in a future release.

For golang/go#45458
Fixes golang/go#50792

Change-Id: I93aa251a7afe7b329a3d3faadc0c5d6388b1f0e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/382460


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>