Revert "net: add missing error check in test"

This reverts commit ec521467.

Reson for revert: The test cases using slowDst4 and slowDst6 are
fragile. We need to find out a better approach to the trick on the IP
routeability.

Change-Id: I544453886e809d1c7b339673d8f1d5bdef357147
Reviewed-on: https://go-review.googlesource.com/c/163919Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dial_test.go

@@ -338,11 +338,6 @@ func TestDialParallel(t *testing.T) {
 		if c != nil {
 			c.Close()
 		}
-		if tt.expectOk && err != nil {
-			t.Errorf("#%d (cancel): got %v; want nil", i, err)
-		} else if !tt.expectOk && err == nil {
-			t.Errorf("#%d (cancel): got nil; want non-nil", i)
-		}
 		elapsed = time.Now().Sub(startTime)
 		if elapsed > 100*time.Millisecond {
 			t.Errorf("#%d (cancel): got %v; want <= 100ms", i, elapsed)