Dockerfile: Update the reason to build Go from bootstrap version

The change https://go-review.googlesource.com/c/playground/+/337010 made
the second reason no longer valid as building standard library with
faketime happen in the final stage and doesn't require to build from
bootstrap version.

Change-Id: I3a7331d235497e9794e2c44c0ebf42cf4356e6fd
Reviewed-on: https://go-review.googlesource.com/c/playground/+/679415
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

Dockerfile

diff --git a/Dockerfile b/Dockerfile
index 8779e59..3a22be2 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -2,11 +2,9 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-# The playground builds Go from a bootstrap version for two reasons:
-# - The playground deployment is triggered before the artifacts are
-#   published for the latest version of Go.
-# - The sandbox builds the Go standard library with a custom build
-#   flag called faketime.
+# The playground builds Go from a bootstrap version because
+# the playground deployment is triggered before the artifacts are
+# published for the latest version of Go.
 
 # GO_VERSION is provided by Cloud Build, and is set to the latest
 # version of Go. See the configuration in the deploy directory.