lockdep: do not use $CEPH_LOCKDEP for g_lockdep

* a non-zero CEPH_LOCKDEP brings ceph down because g_lockdep_ceph_ctx
  is still being constructed when dout_impl() dereferences it.
* fix a typo in comment.
* remove dead code.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Merge pull request #2983 from tchaikov/wip-fix-lock-dep

lockdep: do not use $CEPH_LOCKDEP for g_lockdep

Reviewed-by: Sage Weil <sage@redhat.com>