mgmt/mcumgr: Remove mgmt_ntoh_hdr and mgmt_hton_hdr

These two-liners have been only used by SMP protocol processing,
and there is no reason to have them available as public functions.
Code from these functions have been moved directly where they
have been used and they have been removed.

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