that wasn't a memory leak

the table falls out of context at the end of the loop anyway
so the GC will delete it when the loop ends. No reason to delete it manually.

plants_lib/init.lua

@@ -278,7 +278,6 @@ function plantslib:generate_block(minp, maxp, blockseed)
 				end
 			end
 		end
-		surface_nodes.blockhash = nil -- nuke the block cache after using it (prevent a mem leak).
 	end
 end