Clear old tree counter in map array on load of old savegames

See: https://github.com/OpenTTD/OpenTTD/pull/10018

src/saveload/extended_ver_sl.cpp

@@ -175,6 +175,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_INDUSTRY_ANIM_MASK,     XSCF_IGNORABLE_ALL,       1,   1, "industry_anim_mask",        nullptr, nullptr, nullptr        },
 	{ XSLFI_NEW_SIGNAL_STYLES,      XSCF_NULL,                2,   2, "new_signal_styles",         nullptr, nullptr, "XBST,NSID"    },
 	{ XSLFI_SCRIPT_INT64,           XSCF_NULL,                1,   1, "script_int64",              nullptr, nullptr, nullptr        },
+	{ XSLFI_NO_TREE_COUNTER,        XSCF_IGNORABLE_ALL,       1,   1, "no_tree_counter",           nullptr, nullptr, nullptr        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };