Use the Calibre-Editor (T) and edit the CSS
Just search
Code:
font-weight: bold -> font-weight: normal
IMHO DON'T just change all you find, some may be for Headings or other special uses that you might still want.
Play 'match that class'
to be sure you go after the villains only
In code view: ctrl-mouseclick on the word class (in the tag), should take you to that place in the CSS for that series of use