Revert Plan9 workaround

Author: rafael2k

src/net/http/serve_test.go

@@ -4606,11 +4606,7 @@ func testServerEmptyBodyRace(t *testing.T, mode testMode) {
 			if err != nil {
 				// Try to deflake spurious "connection reset by peer" under load.
 				// See golang.org/issue/22540.
-				deflake := 10 * time.Millisecond
-				if runtime.GOOS == "plan9" {
-					deflake = 100 * time.Millisecond
-				}
-				time.Sleep(deflake)
+				time.Sleep(10 * time.Millisecond)
 				res, err = cst.c.Get(cst.ts.URL)
 				if err != nil {
 					t.Error(err)