It depends how the bold is defined in your document, there are several ways to do it. At a guess, I would think your doc is using <b> tags for bold. If so, you can use search and replace feature while doing a mobi->mobi conversion. Use this search:
Replace with nothing
If that doesn't work then you can probably fix it by converting the mobi->epub, fix the css in Sigil or by using Calibre's tweak epub feature, then convert back to mobi.