dccp ccid-2: Remove old infrastructure

This removes
 * functions for which updates have been provided in the preceding patches and
 * the @av_vec_len field - it is no longer necessary since the buffer length is
   now always computed dynamically;
 * conditional debugging code (CONFIG_IP_DCCP_ACKVEC).

The reason for removing the conditional debugging code is that Ack Vectors are 
an almost inevitable necessity - RFC 4341 says that for CCID-2, Ack Vectors must
be used. Furthermore, the code would be only interesting for coding - after some 
extensive testing with this patch set, having the debug code around is no longer
of real help.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>