btrfs: remove raid-stripe-tree encoding field from stripe_extent

Remove the encoding field from 'struct btrfs_stripe_extent'. It was
originally intended to encode the RAID type as well as if we're a data
or a parity stripe.

But the RAID type can be inferred form the block-group and the data vs.
parity differentiation can be done easier with adding a new key type
for parity stripes in the RAID stripe tree.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>