btrfs: add and use helper for unlinking inode during log replay

During log replay there is this pattern of running delayed items after
every inode unlink. To avoid repeating this several times, move the
logic into an helper function and use it instead of calling
btrfs_unlink_inode() followed by btrfs_run_delayed_items().

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>