btrfs: remove unused btrfs_io_stripe::length

First added (but not effectively used) in 02c372e1 ("btrfs: add
support for inserting raid stripe extents"). The structure is
initialized to zeros so the only use in btrfs_insert_one_raid_extent()

    u64 length = bioc->stripes[i].length;
    struct btrfs_raid_stride *raid_stride = &stripe_extent->strides[i];

    if (length == 0)
            length = bioc->size;

the 'if' always happens.

Last use in 4016358e ("btrfs: remove unused variable length in
btrfs_insert_one_raid_extent()") was an obvious cleanup. It seems to be
safe to remove, raid-stripe-tree works without using it since 6.6.

This was found by tool https://github.com/jirislaby/clang-struct

 .

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