Codechange: use "[[maybe_unused]]" instead of a wide variety of other ways we had

While at it, replace OTTD_ASSERT with WITH_ASSERT, as this
is always set if assert() is valid. No matter if NDEBUG is set
or not.

src/tunnelbridge_cmd.cpp

@@ -1846,8 +1846,7 @@ static void ChangeTileOwner_TunnelBridge(TileIndex tile, Owner old_owner, Owner
 		if (tt == TRANSPORT_RAIL) {
 			/* Since all of our vehicles have been removed, it is safe to remove the rail
 			 * bridge / tunnel. */
-			CommandCost ret = DoCommand(tile, 0, 0, DC_EXEC | DC_BANKRUPT, CMD_LANDSCAPE_CLEAR);
-			(void)ret; // assert only
+			[[maybe_unused]] CommandCost ret = DoCommand(tile, 0, 0, DC_EXEC | DC_BANKRUPT, CMD_LANDSCAPE_CLEAR);
 			assert(ret.Succeeded());
 		} else {
 			/* In any other case, we can safely reassign the ownership to OWNER_NONE. */