If you use Firefox, then, using a plain text editor (create and) edit ~/.mozilla/Your_unique_profile/firefox/chrome/userContent.css to have this:

@namespace html url(http://www.w3.org/1999/xhtml);
@-moz-document domain(askubuntu.com) { 
SPAN.hotbg.unread-count { display: none !important }
}

This works on the main site.

If you use Firefox, then, using a plain text editor (create and) edit ~/.mozilla/Your_unique_profile/firefox/chrome/userContent.css to have this:

@namespace html url(http://www.w3.org/1999/xhtml);
@-moz-document domain(askubuntu.com) { 
SPAN.hotbg.unread-count { display: none !important }
}

This works on the main site.

To do the same in Google Chrome, you can try adding

SPAN.hotbg.unread-count { display: none !important }

to Custom.css which should located in ~/.config/google-chrome/Default/User StyleSheets. If you have more than one profile, you need to specify a particular in place of Default.

This is not a "clean" approach because it isn't site-specific. If other sites also use SPAN.hotbg.unread-count, whatever that code displays in those sites will be hidden as well.

One way to get site-specific styles in Google Chrome is to use the Stylish extension.