py/sequence: Remove unused len argument from mp_seq_extract_slice.

Also put this function inside the `MICROPY_PY_BUILTINS_SLICE` guard,
because it's only usable when that option is enabled.

Signed-off-by: Damien George <damien@micropython.org>

py/obj.h

@@ -1258,7 +1258,7 @@ bool mp_seq_cmp_bytes(mp_uint_t op, const byte *data1, size_t len1, const byte *
 bool mp_seq_cmp_objs(mp_uint_t op, const mp_obj_t *items1, size_t len1, const mp_obj_t *items2, size_t len2);
 mp_obj_t mp_seq_index_obj(const mp_obj_t *items, size_t len, size_t n_args, const mp_obj_t *args);
 mp_obj_t mp_seq_count_obj(const mp_obj_t *items, size_t len, mp_obj_t value);
-mp_obj_t mp_seq_extract_slice(size_t len, const mp_obj_t *seq, mp_bound_slice_t *indexes);
+mp_obj_t mp_seq_extract_slice(const mp_obj_t *seq, mp_bound_slice_t *indexes);
 
 // Helper to clear stale pointers from allocated, but unused memory, to preclude GC problems
 #define mp_seq_clear(start, len, alloc_len, item_sz) memset((byte *)(start) + (len) * (item_sz), 0, ((alloc_len) - (len)) * (item_sz))