btrfs: don't check for btrfs_device::bdev in btrfs_end_bio

btrfs_map_bio ensures that all submitted bios to devices have valid
btrfs_device::bdev so this check can be removed from btrfs_end_bio. This
check was added in june 2012 597a60fa ("Btrfs: don't count I/O
statistic read errors for missing devices") but then in October of the
same year another commit de1ee92a

 ("Btrfs: recheck bio against
block device when we map the bio") started checking for the presence of
btrfs_device::bdev before actually issuing the bio.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>