Update flood_map.cpp

Removed pointless 68K stuff.
Removed numerous uses of "struct (struct type)" as lvars.
Inverted if statement at end of flood_map.
Inverted if statement in body of reverse_flood_map
Inverted if in choose_random_flood_node
Removed a commented-out dprintf in add_node. Probably been there for over a decade.
Inverted if in add_node
Replaced cast from NULL with nullptr
Made more lvars auto.
Inverted second if in add_node.
replaced node_count += 1 with ++node_count