Partially revert "Add ddt, ddt_entry, and l2arc_hdr caches"

This reverts only the l2arc_hdr part of commit
ecf3d9b8e63e5659269e15db527380c65780f71a in preparation for the illumos
5497 "lock contention on arcs_mtx" patch which does the same thing
but uses the newer two-level ARC structure following the Illumos 5408
"managing ZFS cache devices requires lots of RAM" patch.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>