os/exec: remove BUG for old and unsupported OS

Nobody uses 10.6 these days anyway.

Fixes #20623

Change-Id: I698c83cbc288082558e34097ff54d1428aed75ec
Reviewed-on: https://go-review.googlesource.com/45171Reviewed-by: Matt Layher <mdlayher@gmail.com>

src/os/exec/exec.go

@@ -91,9 +91,6 @@ type Cmd struct {
 	// ExtraFiles specifies additional open files to be inherited by the
 	// new process. It does not include standard input, standard output, or
 	// standard error. If non-nil, entry i becomes file descriptor 3+i.
-	//
-	// BUG(rsc): On OS X 10.6, child processes may sometimes inherit unwanted fds.
-	// https://golang.org/issue/2603
 	ExtraFiles []*os.File
 
 	// SysProcAttr holds optional, operating system-specific attributes.