jbd[2]: remove references to BUFFER_DEBUG

CONFIG_BUFFER_DEBUG seems to have been removed from the documentation
somewhere around 2.4.15 and seemingly hasn't been available even
longer. It is, however, still referenced at one place from the jbd
code (one is a copy of the other header). Time to clean it up

Signed-off-by: Christoph Egger <siccegge@stud.informatik.uni-erlangen.de>
Signed-off-by: Jan Kara <jack@suse.cz>