Revert "Fix bug #21971: visual glitch of terrains on 2p - Hornshark Island"

Requested by Wintermute: <http://r.wesnoth.org/p573664>

This reverts commit 18282b542fe48614d74090535552ba4a9d430682.
Revert "Make Lit Mushroom Grove stats identical to Mushroom Grove"

Requested by Wintermute: <http://r.wesnoth.org/p573664>

This reverts commit 35a48f29f9134169b249af827730d373909e3562.
Changelog entries for the reversal of the fix for bug #21971

Requested by Wintermute: <http://r.wesnoth.org/p573664>