mgmt/mcumgr/lib: Deprecate FS_MGMT_UL_CHUNK_SIZE

Now the chunk of uploaded file is directly read from the transport
buffer, net_buf, and is no longer copied into intermediate buffer
so the real size of chunk is actually how much the sender could
fit into MCUMGR_BUF_SIZE, less the other fields.

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