To remove a note, chord or rest, position the insertion bar on it and press DELETE. To remove a single note from a chord, position both the insertion bar and the pitch crossbar) on the notehead and press BACKSPACE.
This is similar to an issue that vexed me for a long time, particularly with using the Enter key to enter a note in place of a rest. Yes there is a convoluted keystroke to do it on my MacBook Pro, but if I'm holding down notes on a MIDI keyboard with one hand, I can't do the keystroke sequence because the fn key on a MBP is only on the left side and the Return key is on the right. No one has hands wide enough to handle that. So I ended up getting a cheap USB numeric keypad since it has an Enter key. That solved the problem. But given how common Mac laptops are (I'm still using my 2018 MBP and it's working nicely to this day), you'd think MM would have made such keystroke sequences more functional.