Revert "future-utils: fix gross compilation errors in when_all()"
Revert "future-utils: fix gross compilation errors in when_all()"

This reverts commit 8f95dfaf4b775df0fc3f46cb24ea1f1141cec248. Stil doesn't build.

avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,dreamsxin/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar,cloudius-systems/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar