future: remove bogus when_all() forward declaration
future: remove bogus when_all() forward declaration

Doesn't match the definition, and is unneeded anyway.

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