How can I remove the default generated  tags in view's result?
The easiest thing to do is copy the template files from "sites/all/modules/views/theme/" into you theme (I normally make a "views/" subdirectory).

If you want to make this change for all themes, edit "views-view-unformatted.tpl.php".

If you want to just change that view, edit the view then click on the Theme: Information link.  It will show the template suggestions for the view.  Copy 
If you want to make this change for all themes, copy "views-view-unformatted.tpl.php" to the proper name, and edit that file.

In both cases, make sure you clear the Drupal caches to make sure the system finds the new files.