jbd2: remove unused function '__cp_buffer_busy'

jbd2: remove unused function '__cp_buffer_busy'

The code calling function '__cp_buffer_busy' has been removed, so the
function should also be removed.
silence the warning:
fs/jbd2/checkpoint.c:48:20: warning: unused function '__cp_buffer_busy'

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=5518
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230714025528.564988-4-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>