commit 377224c4("f2fs: don't split checkpoint in fstrim") obsolete
batch mode and related sysfs entry.

Since this testing sysfs node has been deprecated for a long time, let's
remove it.
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>