Revert "More interesting nymph level"

This reverts commit dab26430c4a7d21e718f324246675977e56f191f.
nymph levels - at least trees may be kept
Revert "add Is_nymph_level(x), closes issue #10"

This reverts commit 64855fc8bc03bb8308639ded6e2ef0fab12a14bf to keep savegame compatibility with UnNetHack 4.0.0.