[cybmole]

in my experience, whitespace / softbreak are usually there to separate scenes within chapters, i.e. they correspond to where you'd see a blank line in a paper book; so irreversibly removing all of them without previewing the book may not be a good idea.

if you go the sigil route you could redefine them in CSS rather than remove them completely.

Ps unless the book was originally a PDF, in which case a pdf to epub conversion may have added than between "pages"
in that case I "think" the remove spaces between paragraphs option in calibre look and feel preferences will nuke them.