Revert "hashtable: introduce a small and naive hashtable"

This reverts commit ad49a6701748d14afc2a37ee7a19268c1066653c.
Revert "block,elevator: use new hashtable implementation"

This reverts commit c2053d58c2657f31c8576407d3762408cf5e04cf.
Revert "slub: fix a possible memory leak"

This reverts commit e536381f8177bc07923a318eaa84940bad7a3059.