_content/doc: update generics tutorial to not mention constraints package

Following decision in golang/go#50792

Change-Id: I71d74b4b386aacef5319663c397479cac6feb32f
Reviewed-on: https://go-review.googlesource.com/c/website/+/383694
Reviewed-by: DO NOT USE <iant@google.com>
Reviewed-by: Eli Bendersky <eliben@google.com>

_content/doc/tutorial/generics.md

@@ -468,10 +468,6 @@ Generic Sums with Constraint: 46 and 62.97
 
 Nicely done! You've just introduced yourself to generics in Go.
 
-If you want to keep experimenting, you can try writing the `Number` interface
-in terms of `constraints.Integer` and `constraints.Float`, to allow more
-numeric types.
-
 Suggested next topics:
 
 *   The [Go Tour](https://tour.golang.org/welcome/1) is a great step-by-step