In Sigil, regex search and replace:
search:<p class="calibre3">\s*<b class="calibre1">Page (\d+)</b></p>
replace: (nothing)
Try it on a few first before committing to the replace all...and save before you do!
I would also replace all the <p class="calibre3"> with something that is descriptive. eg <p class="noindent"> or, if it is a basic paragraph type, replace it with just <p>