tipc: Remove unused message header field for requested number of links

Eliminates support for the "number of requested links" field in a neighbor
discovery message. This field was never used and has been removed from
the TIPC 2.0 protocol specification.

Signed-off-by: Allan Stephens <Allan.Stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>