You can customise the toolbar on the Mac, but that button is conveniently not part of the toolbar. The 'toolbar', which can be customised, only contains a few things like sync status, alarms and a redundant additional button to create a new note.
I can't believe someone thought this obnoxious green 'share' button was a good idea, and then doubled down by putting it somewhere that it can't be hidden or removed. It's one thing to have bad taste (taste is subjective, after all); it's another thing to force it on your customers. Do they really think that people are suddenly going to start sharing their notes because this ugly green 'Share' button appeared?
All that time and work redesigning the logo to make it look like a modern software application, and they throw it all away by reintroducing something which looks like it escaped from Windows Vista.