mgmt/mcumgr: Remove buf.c

buf.c have been providing net_buf pool allocator and dealocator
for SMP packets: mcumgr_buf_alloc and mcumgr_buf_free.
The functions have been moved to smp.c and renamed
smp_packet_alloc and smp_packet_free, respectively.

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