Remove BTRFS._resizeArgs()

Without a corresponding _resizefs it has no point in existing.
Resizing btrfs will probably requires "btrfs filesystem resize ..." command,
which doesn't expect those arguments.

Signed-off-by: mulhern <amulhern@redhat.com>
Remove pointless assignment to _formattable in Iso9660FS.

Let it inherit the assignment from DeviceFormat, like all the other
filesystems that are not formattable.

Signed-off-by: mulhern <amulhern@redhat.com>