Remove old ShowSelectedLightmap debug command as it was unsafe and not publicly exposed
#rb michael.noland
#ROBOMERGE-OWNER: ryan.gerleve
#ROBOMERGE-AUTHOR: ben.zeigler
#ROBOMERGE-SOURCE: CL 5048006 in //UE4/Release-4.22/... via CL 5048008
#ROBOMERGE-BOT: ENGINE (Main -> Dev-Networking)
[CL 5129382 by ben zeigler in Dev-Networking branch]