I also don't understand the point in removing the CSS. I would keep it and tidy up the formatting to make it cleaner.
I use it all the time and I read alot on my HTC Desire HD's 4.3 inch screen and it doesn't take much effort to fix the formatting (with CSS included) to display correctly.
IF you are intent on removing all tags, then in Sigil there is an option in the Format menu to remove formatting. So highlight the text and then either go to the menu option or use Ctrl+Space to remove the formatting. Just remember this will strip out everything in the selected text (if you select an entire paragraph it will put it to just the <p> </p> tags) so you will need to add back in anything that you want to retain manually.