sockbuf: remove unused mbuf counter and cluster counter

With M_EXTPG mbufs these two counters already do not represent the
reality. As we are moving towards protocol independent socket buffers,
which may not even use mbufs at all, the counters become less and less
relevant. The only userland seeing them was 'netstat -x'.

PR: 264181 (exp-run)
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D35334