net: sock: remove dead cgroup methods from struct proto

The cgroup methods are no longer used after baac50bb ("net:
tcp_memcontrol: simplify linkage between socket and page counter").
The hunk to delete them was included in the original patch but must
have gotten lost during conflict resolution on the way upstream.

Fixes: baac50bb ("net: tcp_memcontrol: simplify linkage between socket and page counter")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>