ubifs: journal: Remove wrong comment

In the description of reserve_space() it is claimed that write_node()
and write_head() unlock the journal head. This is not true and has never
been true. All callers of write_node() and write_head() call
release_head() themselves. Remove the wrong comment.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>