syscall: remove a wrong comment in Clearenv

The comment being removed was added by commit ff31738
(which predates Gerrit and Rietveld, so no CL link), and
at the time it made sense.

Since CL 148370043 (and up to the current implementation of Clearenv)
the env map, which is populated by copyenv, is actually used, so the
comment is no longer valid.

It is also misleading, so it's best to remove it.

Change-Id: I8bd2e8bca6262759538e5bcbd396f0c71cca6a4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/635078
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

src/syscall/env_unix.go

@@ -124,7 +124,7 @@ func Setenv(key, value string) error {
 }
 
 func Clearenv() {
-	envOnce.Do(copyenv) // prevent copyenv in Getenv/Setenv
+	envOnce.Do(copyenv)
 
 	envLock.Lock()
 	defer envLock.Unlock()