libnvdimm/btt: Remove unnecessary code in btt_freelist_init

We call btt_log_read() twice, once to get the 'old' log entry, and again
to get the 'new' entry. However, we have no use for the 'old' entry, so
remove it.

Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>