ext2: remove the redundant comment for ext2_export_ops

The comment is outdated and isn't particularly informative anyway - NULL
meaning the default behavior is very common in kernel. And we really set about
half of entries. So remove the whole comment for ext2_export_ops.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>