internal/goexperiment: remove unused range experiment

This experiment was added mistakenly in https://go.dev/cl/537980,
probably as the result of a bad rebase on https://go.dev/cl/539277.

Change-Id: Ia3fc75725535ac70a2e6dd655440a2f515405c48
Reviewed-on: https://go-review.googlesource.com/c/go/+/580375
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

Author: prattmic

src/internal/goexperiment/flags.go

@@ -116,7 +116,4 @@ type Flags struct {
 
 	// RangeFunc enables range over func.
 	RangeFunc bool
-
-	// Range enables range over int and func.
-	Range bool
 }