If you mean the small tool tip that shows the time when you hover on the date in the meta info line, it comes from the Title attribute of the link.
I can think of no way to remove an html attribute with CSS, I believe it would have to be done in the template which means a child theme and PHP editing.
Unless a filter could do the trick but @weaver would have to comment on that.
I had same issue, as well as not wanting the post date to be a hyperlink.
I added Posted on %month% %day%, %year% in the box for Top Post Info Line under Main Options > Post Specifics > Custom Post Info Lines.
Though not necessary since the above replaced the whole post date, I did check off Hide Post Date under Main Options > Post Specifics > Post Meta Info Areas.