Revert "fetch credientical from Driver.cfg (sql-machine-learning#47)" (…

…sql-machine-learning#48)

This reverts commit c7abf14.

driver.go

@@ -12,14 +12,12 @@ func init() {
 }
 
 // impls database/sql/driver.Driver
-type Driver struct {
-	cfg *Config
-}
+type Driver struct{}
 
 func (d Driver) Open(dsn string) (driver.Conn, error) {
-	var err error
-	if d.cfg, err = ParseDSN(dsn); err != nil {
+	cfg, err := ParseDSN(dsn)
+	if err != nil {
 		return nil, err
 	}
-	return &odpsConn{&http.Client{}, d.cfg}, nil
+	return &odpsConn{&http.Client{}, cfg}, nil
 }