Delete the remains of rmgen/utilityfunctions.js, refs #4804 by
cleaning up the createForests function.

Remove g_numStragglerTrees and clForest references from the rmgen library.
Compute the number of forest- and straggler trees in a new function instead of hiding it and obstructing the placement code.
Remove unused numMultiplier argument.

Replace duplication of these three functions with calls to them, refs #4805.
Remove duplicate random biome JSON constants of the Islands, Migration and Snowflake Searocks map.
There is significant partial createForests duplication that should be unified eventually.