removed all code that tries to disable default trees because the minetest dev

removed all code that tries to disable default trees because the minetest dev
team is being completely obstinant about letting me properly disable them from
this mod.

Instead, made beech trees respond to enable flags in settings.lua (disabled by
default).  If you want beeches, turn that flag on in settings.lua and remove
the "trees" flag from yourworld/map_meta.txt in the mg_flags line.