Make all trees spawn again

[cx384]

Most spawn by water source trees (including rubber trees) will never spawn.
(spawn_by and num_spawn_by is only for the 8 neighboring nodes at y coordinate -1.)
This is horrible if you actually want to use this mod, it has been a long time since I last did.
Fixes #48 #33 and forum posts.

Only for trees that grow in water (not even for palms), it would make sense to use spawn by water.
So I removed most of it. For palms, restricting y_max works better.

I tested it (in MTG) and found every tree in a reasonable about of time. Also, the trees are not too common, imo.

It's not perfect, a better solution would use biomes in the decoration definition depending on their heat_point and humidity_point, but I don't want to figure out numbers and deal with compatibility problems.
So with this PR at least all trees (including rubber trees) will spawn again.

Moreover, note that the trees still don't spawn on default:dirt_with_dry_grass, default:dry_dirt_with_dry_grass and default:dirt_with_snow (the latter would require special snow covered saplings.)
Feel free to make improvements

[wsor4035]

seems legit. leaving this open for a day before merging in case anyone wants to comment