Remove the `scm_tc_free_cell' SMOB type.

Remove the `scm_tc_free_cell' SMOB type.

* libguile/deprecated.h (SCM_FREEP, SCM_NFREEP): Changed to constants.

* libguile/gc.c (scm_i_tag_name): Remove reference to
  `scm_tc_free_cell'.

* libguile/gc.h (SCM_FREE_CELL_CDR, SCM_SET_FREE_CELL_CDR): Remove.

* libguile/smob.c (free_print): Remove.
  (scm_smob_prehistory): Don't create the "free" SMOB type.

* libguile/struct.c (struct_finalizer_trampoline): Use a bare
  `scm_tc3_struct' tag for finalized structs instead of
  `scm_tc_free_cell'.

* libguile/tags.h (scm_tc_free_cell): Remove.