Remove trenches from map generator (cyisfor) (glitches: read below)

This causes glitches in very high cliffs by not making mud "flow" down from
them. Those are quite rare currently because of the lame height differences
generated.

src/mapgen.cpp

@@ -2059,13 +2059,14 @@ void make_block(BlockMakeData *data)
 				
 				bool old_is_water = (n->getContent() == c_water_source);
 				// Move mud to new place
-				if(!dropped_to_unknown)
+				if(!dropped_to_unknown) {
 					*n2 = *n;
 					// Set old place to be air (or water)
 					if(old_is_water)
 						*n = MapNode(c_water_source);
 					else
 						*n = MapNode(CONTENT_AIR);
+				}
 
 				// Done
 				break;