Update mad_dict.c

Fall back from PR 1175

For proper slicing in the cavities volt needs to be added in the list of attribute to slide, but also fill correctly the list of cavities in the sequence structure

Author: rdemaria

src/mad_dict.c

@@ -2034,7 +2034,6 @@ const char *const_element_def =
 "harmon   = [i, 0],  "
 "nbin     = [i, 0],  "
 "magnet   = [i, 0],  "
-"slice    = [i, 1],  "
 "slot_id  = [i, 0], "
 "assembly_id = [i, 0], "
 "model           = [i, -1], "