f2fs: remove unnessary comment in __may_age_extent_tree

This comment make no sense and is in the wrong place, so let's
remove it.

Signed-off-by: Qi Han <hanqi@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>