Yup, those markers give the ability to arrange any set objects to a desired season. For a residential lot it can be a tad weird, but it could give some inspiration to do your decor based on a specific seasons.
To remove the seasonal marker (go to 4, since you already found it):
1. Go in Buy/Build mode
2. Activate cheatconsole (CTRL+SHFT+C) and type in: "testingcheatsenabled true" (without quotes)
3. Activate again the cheatconsole and type in: "buydebug on" (without quotes)
4. you might need to switch from Buy Mode to Build mode and back again to see the Seasonal Marker, if you see it, hold down CTRL+SHFT and click on that shiny ball, then from the pie select "Delete It".
Unsure what will happen with the furniture that's been placed on specific seasonal triggers, best hunch is that they will just be dropped in common construction mode.
Good luck!