net: remove old comment

The memory model has been clarified since. This is legal and doesn't
need justification.

Change-Id: I60f9938503f86f52bb568ca1a99ac721ee72cee5
Reviewed-on: https://go-review.googlesource.com/8913Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/net/net.go

@@ -419,10 +419,6 @@ func genericReadFrom(w io.Writer, r io.Reader) (n int64, err error) {
 
 var threadLimit = make(chan struct{}, 500)
 
-// Using send for acquire is fine here because we are not using this
-// to protect any memory. All we care about is the number of goroutines
-// making calls at a time.
-
 func acquireThread() {
 	threadLimit <- struct{}{}
 }