I'm not sure about Tableau Public layout.
But usually, to get rid of the borders on a sheet you can do the following :
1 / In the main menu go to Format -> Borders ( alternatively you can right click on the sheet then click 'Format' )
2 / On the left side you'll see 'Column Divider' under 'Sheet' Tab. Change 'Pane' from line to 'None'
I hope this was helpful !
Just try to click anywhere else but not on the image.