remove redundant, unnecessary instance size from struct vtables

remove redundant, unnecessary instance size from struct vtables

* libguile/struct.h (scm_struct_i_size): Remove this shared field -- I
  mean, the slot is still there, but it's only used for flags.

* libguile/goops.h (SCM_SET_CLASS_INSTANCE_SIZE):
* libguile/goops.c (scm_sys_inherit_magic_x, scm_make_class): Remove
  uses and definition of SCM_SET_CLASS_INSTANCE_SIZE. Light structs used
  it, but you have that info in the layout; and foreign classes used it,
  but that is going away soon anyway :)