mgmt/mcumgr: Remove internal functions form Zephyr header

The commit removes declarations of:
 zephyr_smp_rx_req, zephyr_smp_alloc_rsp, zephyr_smp_free_buf
from include/zephyr/mgmt/mcumgr/smp.h, as these are MCUMgr internal
functions used in SMP processing and should be not exposed
from header file that provides interface for SMP transports.
The declarations have been moved to smp_internal.h, which is
visible within MCUMgr.

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