remove index file

fix https://github.com/chrislusf/seaweedfs/issues/793

weed/storage/volume_read_write.go

@@ -42,9 +42,7 @@ func (v *Volume) Destroy() (err error) {
 	}
 	v.Close()
 	os.Remove(v.FileName() + ".dat")
-	if v.nm != nil {
-		err = v.nm.Destroy()
-	}
+	os.Remove(v.FileName() + ".idx")
 	os.Remove(v.FileName() + ".cpd")
 	os.Remove(v.FileName() + ".cpx")
 	os.Remove(v.FileName() + ".ldb")