Remove some duplicate initialisation of struct dm_pool_metadata.

These pmd fields are initialised by both:
  __format_metadata's calls to dm_btree_empty
  __write_initial_superblock + __begin_transaction
Signed-off-by: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>