x509roots/fallback: drop obsolete build constraint

Back when Go 1.20 and 1.19 were supported, the go1.20 build constraint
was there to establish 1.20 as the minimum for the package, because it
requires an API that was added only in Go 1.20 and there was no way to
make it work with 1.19's APIs.

By now only Go 1.24 and 1.23 are supported, and the minimum is set via
the go directive in go.mod.

For golang/go#57792.

Change-Id: Ie4d37d34993374cdd380667930ee667af97eeccb
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/649716
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

x509roots/fallback/bundle.go

@@ -1,7 +1,5 @@
 // Code generated by gen_fallback_bundle.go; DO NOT EDIT.
 
-//go:build go1.20
-
 package fallback
 
 import "crypto/x509"

x509roots/fallback/fallback.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.20
-
 // Package fallback embeds a set of fallback X.509 trusted roots in the
 // application by automatically invoking [x509.SetFallbackRoots]. This allows
 // the application to work correctly even if the operating system does not

x509roots/gen_fallback_bundle.go

@@ -27,8 +27,6 @@ import (
 
 const tmpl = `// Code generated by gen_fallback_bundle.go; DO NOT EDIT.
 
-//go:build go1.20
-
 package fallback
 
 import "crypto/x509"