net: cdc_ncm: remove descriptor pointers

header_desc was completely unused and union_desc was never used
outside cdc_ncm_bind_common.

Cc: Alexey Orishko <alexey.orishko@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>