I think I may have accidentally stumbled upon a work around for this annoying issue while looking for a way to remove those guidelines when “Wrap to Page” is checked in the app’s settings. While looking through the other options in Settings, I simply unchecked “Show ruler”, and to my surprise the horizontal scrollbar disappeared. Unchecking the option in the app's settings results in the scrollbar not being present on future New Documents, while clicking on Format > Text in the Menu Bar for an open document that already has the unneeded horizontal bar, and again simply unchecking the "Show Ruler" option removes the horizontal bar.
In both situations the vertical bar remains visible for me (I have my Mac global settings to show scrollbars "Always". Other than no longer having the ruler constantly visible (which I virtually never use anyway), I can still "Show" it if I need to change the margins and/or tab indent settings for the document, and then turn the Ruler back off again.
Am I missing any major (or minor) drawbacks to this “fix” that would make it not worthwhile? Let me know if there are. Thanks.