Hi,
Most of that stuff in the skin you do not want to remove. How a site "looks" is all based on html and css, if you want it to look plain then make a plain skin. I would not approach the goal by thinking what can I remove from the skin other than css changes.
Yes, you can remove some stuff, but if you remove login link and logout link you lose that functionality, how will the user know where to login or how to logout.
SiteMapDataSource is required to populate the menu and is not itself visible
ScriptManager is a needed component of MS AJAX, its not a visible element
SiteMenu shows the links for user created pages, ie content system pages
divLeft, divRight, divCenter are all needed for the included skins and the code behind for the master page expects them and apllies css classes to them based on whether any content instances exist that are supposed to be in those sections.
If mojoPortal meets your needs use it if not don't.
Best,
Joe