You can't use the Lot Editor to remove the lamppost from the ped-mall puzzle piece, but it should be doable with the Reader. Make a backup of the "NetworkAddonMod5.dat" file and then open it with the Reader. Click on "Fill the list" and then click on the very first exemplar file in the list. It has the name "Ped Mall Tiles_CentrePole". Right-click on the "LotConfigPropertyLotObject" line, and select "Delete property" from the pop-up menu. Now right-click again and select "Reindex LotConfig". Finally, save the file. Most likely, you will have to bulldoze and re-plop the puzzle pieces to get rid of the lampposts, but after doing so, they shouldn't show the lamppost anymore.