btrfs: remove redundant level reset in btrfs_del_items()

btrfs: remove redundant level reset in btrfs_del_items()

When btrfs_del_items() empties a leaf, it deletes the leaf unless it's
the root node. For the root leaf case, the code used to reset its level
to 0 via btrfs_set_header_level(). This is redundant as leaf nodes
always have level == 0.

Remove the unnecessary level assignment and invert the conditional to
handle only the non-root leaf deletion. The root leaf is correctly left
as-is.

Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>