cmd/go/internal/test: remove redundant if statement

Just return the result of the function call as they are
both functionally equivalent.

Change-Id: Ia7847c9b018475051bf6f7a7c532b515bd68c024
Reviewed-on: https://go-review.googlesource.com/90375
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/test/test.go

@@ -1805,11 +1805,7 @@ func writeTestmain(out string, t *testFuncs) error {
 	}
 	defer f.Close()
 
-	if err := testmainTmpl.Execute(f, t); err != nil {
-		return err
-	}
-
-	return nil
+	return testmainTmpl.Execute(f, t)
 }
 
 type testFuncs struct {