net: enable TestCancelRequestWithChannelBeforeDo on Plan 9

This issue has been fixed in CL 31390.

Fixes #11476.

Change-Id: I6658bda2e494d3239d62c49d0bd5d34a36b744d0
Reviewed-on: https://go-review.googlesource.com/31394Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/transport_test.go

@@ -1696,12 +1696,6 @@ func testCancelRequestWithChannelBeforeDo(t *testing.T, withCtx bool) {
 	defer ts.Close()
 	defer close(unblockc)
 
-	// Don't interfere with the next test on plan9.
-	// Cf. https://golang.org/issues/11476
-	if runtime.GOOS == "plan9" {
-		defer time.Sleep(500 * time.Millisecond)
-	}
-
 	tr := &Transport{}
 	defer tr.CloseIdleConnections()
 	c := &Client{Transport: tr}