How can I remove ridges from a concrete shower pan?
You probably don't need to concern yourself.  The thinset plus tile should smooth out the imperfections just fine.

If you really want to remove them, you'll probably need to use a concrete grinder.  But that's pretty expensive.  As a "lightweight" alternative, a 4" angle grinder with a  masonry wheel would knock the top off of the ridges.  But it would take a lot of time and make a lot of mess to really remove much material that way.