Clean up old Go 1.10 build tags

pgx requires Go modules which requires at least Go 1.11 so there is no
use in build tags to support older Go versions.

stdlib/sql_test.go

@@ -19,6 +19,15 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func openDB(t testing.TB) *sql.DB {
+	config, err := pgx.ParseConfig(os.Getenv("PGX_TEST_DATABASE"))
+	if err != nil {
+		t.Fatalf("pgx.ParseConnectionString failed: %v", err)
+	}
+
+	return stdlib.OpenDB(*config)
+}
+
 func closeDB(t testing.TB, db *sql.DB) {
 	err := db.Close()
 	if err != nil {