fs/squashfs: remove useless chmod

fs/squashfs: remove useless chmod

When that was added (in 975e30b, fs/squashfs: fix image file
permissions), the reasons were not quite explicit.

We are now forcing the umask, and various tests have shown that the mode
on the generated image file are correct without the chmod.

Remove it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>