go: remove misleading comment in go.env

See #202030 and golang/go#70949.

In particular, our current `go.env` contains

    # Automatically download newer toolchains as directed by go.mod files.
    # See https://go.dev/doc/toolchain for details.
    GOTOOLCHAIN=local

The comment about automatically downloading newer toolchains applies
only whenever `GOTOOLCHAIN=auto`, but we change that to
`GOTOOLCHAIN=local`. Therefore, let's get rid of the comment that no
longer applies after our `GOTOOLCHAIN` change.

Formula/g/go.rb

@@ -63,7 +63,11 @@ class Go < Formula
   end
 
   def install
-    inreplace "go.env", /^GOTOOLCHAIN=.*$/, "GOTOOLCHAIN=local"
+    inreplace "go.env" do |s|
+      # Remove misleading comment about automatically downloading newer toolchains.
+      s.gsub!(/^# Automatically download.*$/, "")
+      s.gsub!(/^GOTOOLCHAIN=.*$/, "GOTOOLCHAIN=local")
+    end
 
     (buildpath/"gobootstrap").install resource("gobootstrap")
     ENV["GOROOT_BOOTSTRAP"] = buildpath/"gobootstrap"