fs/squashfs: remove unused declarations

fs/squashfs: remove unused declarations

This patch removes a number of struct and macro declaration that
were found through `git-grep` to be unused. Most of those are
related to compressor options and super block flags.

For reading a SquashFS image, we do not need the compressor options
or the flags. Those only encode settings used for packing the image,
mksquashfs uses them when appending data to an existing image. The
kernel implementation does not touch those, and we don't need them
either.

Signed-off-by: David Oberhollenzer <goliath@infraroot.at>