There is a “remove duplicate” option in the MIDI menu, but it may only work within a single track.
I would simply go to your track #! (or copy it to #3 first if you want to keep #1 intact), open the key editor and remove or mute the low notes. Most piano parts have reasonably well-defined L/R parts, but you might need to be a bit careful, and do some cleanup after the first delete. But it shouldn’t take long.