memcache: remove unused subdevice::_length
memcache: remove unused subdevice::_length

Noticed by clang.

stamhe/scylla,scylladb/scylla,joerg84/seastar,dwdm/seastar,senseb/scylla,xtao/seastar,scylladb/scylla,bowlofstew/seastar,senseb/scylla,raphaelsc/seastar,gwicke/scylla,avikivity/seastar,eklitzke/scylla,bzero/seastar,phonkee/scylla,tempbottle/scylla,mixja/seastar,scylladb/seastar,guiquanz/scylla,dwdm/scylla,senseb/scylla,phonkee/scylla,aruanruan/scylla,scylladb/seastar,slivne/seastar,scylladb/scylla-seastar,bowlofstew/seastar,flashbuckets/seastar,wildinto/scylla,syuu1228/seastar,gwicke/scylla,rluta/scylla,glommer/scylla,scylladb/seastar,jonathanleang/seastar,syuu1228/seastar,dreamsxin/seastar,koolhazz/seastar,eklitzke/scylla,norcimo5/seastar,jonathanleang/seastar,bowlofstew/seastar,asias/scylla,bowlofstew/scylla,wildinto/scylla,hongliangzhao/seastar,acbellini/scylla,linearregression/scylla,kjniemi/seastar,duarten/scylla,avikivity/scylla,stamhe/seastar,scylladb/scylla,sjperkins/seastar,linearregression/seastar,asias/scylla,flashbuckets/seastar,koolhazz/seastar,shaunstanislaus/scylla,rentongzhang/scylla,stamhe/seastar,sjperkins/seastar,stamhe/seastar,guiquanz/scylla,koolhazz/seastar,aruanruan/scylla,victorbriz/scylla,scylladb/scylla-seastar,slivne/seastar,bzero/seastar,norcimo5/seastar,scylladb/scylla-seastar,kjniemi/scylla,asias/scylla,wildinto/seastar,cloudius-systems/seastar,stamhe/scylla,dreamsxin/seastar,joerg84/seastar,duarten/scylla,avikivity/scylla,capturePointer/scylla,syuu1228/seastar,raphaelsc/scylla,cloudius-systems/seastar,jonathanleang/seastar,guiquanz/scylla,acbellini/scylla,printedheart/seastar,justintung/scylla,chunshengster/seastar,kangkot/scylla,linearregression/seastar,aruanruan/scylla,gwicke/scylla,glommer/scylla,chunshengster/seastar,kangkot/scylla,phonkee/scylla,cloudius-systems/seastar,dwdm/scylla,slivne/seastar,capturePointer/scylla,raphaelsc/seastar,printedheart/seastar,tempbottle/seastar,mixja/seastar,leejir/seastar,ducthangho/imdb,shyamalschandra/seastar,dwdm/scylla,xtao/seastar,ducthangho/imdb,tempbottle/scylla,dwdm/seastar,justintung/scylla,leejir/seastar,hongliangzhao/seastar,dreamsxin/seastar,duarten/scylla,avikivity/seastar,flashbuckets/seastar,victorbriz/scylla,avikivity/seastar,kjniemi/seastar,hongliangzhao/seastar,rluta/scylla,tempbottle/seastar,capturePointer/scylla,mixja/seastar,acbellini/seastar,respu/scylla,acbellini/seastar,leejir/seastar,linearregression/scylla,sjperkins/seastar,rluta/scylla,printedheart/seastar,kjniemi/seastar,glommer/scylla,kangkot/scylla,respu/scylla,eklitzke/scylla,ducthangho/imdb,avikivity/scylla,linearregression/scylla,shyamalschandra/seastar,wildinto/seastar,victorbriz/scylla,dwdm/seastar,bzero/seastar,tempbottle/seastar,rentongzhang/scylla,acbellini/seastar,bowlofstew/scylla,tempbottle/scylla,anzihenry/seastar,linearregression/seastar,raphaelsc/scylla,joerg84/seastar,raphaelsc/seastar,wildinto/scylla,raphaelsc/scylla,respu/scylla,kjniemi/scylla,stamhe/scylla,justintung/scylla,shaunstanislaus/scylla,anzihenry/seastar,rentongzhang/scylla,acbellini/scylla,shaunstanislaus/scylla,kjniemi/scylla,chunshengster/seastar,shyamalschandra/seastar,bowlofstew/scylla,anzihenry/seastar,norcimo5/seastar,scylladb/scylla,xtao/seastar,wildinto/seastar