As Idolse said, it depends how the bold is defined in your mobi.
You could try a Calibre mobi to mobi conversion with the following in the ExtraCSS box on the
Convert - Look&Feel page:
Code:
body, div, p {font-weight: normal;}
This is a rather simplistic approach but you might be lucky and at least it's easy to try.
Caveat: You might like to save your original mobi before trying it, just in case, as Calibre will overwrite the old mobi with the new mobi.