mgmt/mcumgr/lib: Remove TinyCBOR state objects from mgmt_ctxt

Removes no longer needed elements of struct mgmt_ctxt.
Removes mgmt_ctxt_init, mgmt_streamer_init_reader and init_reader
from struct mgmt_streamer_cfg as they are no longer needed.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>