Revert "cgroup: use new hashtable implementation"

This reverts commit 664107c1e16a5eb19c29534cfc814c0e331a2ee6.
Revert "block,elevator: use new hashtable implementation"

This reverts commit 0c5194f013d0c3dc2af44f0edc2b83f5eb56cc8a.
Revert "workqueue: use new hashtable implementation"

This reverts commit 013cdd5627397dcdb6ad5a8838d27d49799337b8.
Revert "hashtable: introduce a small and naive hashtable"

This reverts commit b7d3bf00e629cdfdbade01b6534cff2aec09b926.