L-System trees: Remove hardcoded use of 'mapgen_dirt' alias (#9931)

L-System trees: Remove hardcoded use of 'mapgen_dirt' alias (#9931)

Games often and increasingly do not use this mapgen alias, as it
is only required for Mapgen V6. Such games were triggering the
recently added error message.

Even if this mapgen alias was defined, dirt nodes placed under a
wide trunk were inconsistent with biomes that do not use dirt
surface nodes.

Place trunk nodes below a wide trunk instead of 'mapgen_dirt'.
On sloping ground, the trunk then extends down to the surface,
instead of the surface rising up to meet the trunk. This looks
more natural and does not alter the terrain.