Is there any method to clean the website unused resources like...
this thing require, when website is completed and ready for deployment.
I'm not sure about doing it in Visual Studio other than doing a Find In Solution for each CSS rule / Skin / Image.
To find unused CSS rules, you could use the PageSpeed add-on for the Firebug add-on for Firefox. This will analyse individual pages of your site and give you a load of info on how to speed it up, including removing unused CSS rules. Might be helpful!
There's a firefox add-on Dust-Me Selectors that will find unused CSS selectors.
Be careful with looking the pages once they're rendered, in case selectors are used in code but not rendered in the final pages due to conditions during execution.
I can't think of anything other than an old fashioned find/replace in the code-behinds because a program would have to have syntactical knowledge of the tags, but if they're being built with, they'll probably be strings.