AFFS: clean up dirty flag usage

In 'affs_write_super()': remove ancient and wrong commented code,
remove unneeded 'clean' variable, so the function becomes a bit
cleaner and simpler.

In 'affs_remount(): remove unnecessary SB dirty flag changes.

Tested-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>