Remove unused/empty files

The `compress.sh` file hasn't been used for a long time.
The `.gitmodules` file was emptied as it wasn't used any more.

Just get rid of both of them.

compress.sh

@@ -1,34 +0,0 @@
-#!/bin/bash
-# Copies raspi3.img into compr.img, resulting in many consecutive zero bytes
-# which are nicely compressible.
-
-set -e
-
-qemu-img create -f raw compr.img 1100M
-
-# copy partition table from raspi3.img
-sfdisk --quiet --dump raspi3.img | sfdisk --quiet compr.img
-
-readarray rmappings < <(sudo kpartx -asv raspi3.img)
-readarray cmappings < <(sudo kpartx -asv compr.img)
-
-# copy the vfat boot partition as-is
-set -- ${rmappings[0]}
-rboot="$3"
-set -- ${cmappings[0]}
-cboot="$3"
-sudo dd if=/dev/mapper/${rboot?} of=/dev/mapper/${cboot?} bs=5M status=none
-
-# copy the ext4 root partition in a space-saving way
-set -- ${rmappings[1]}
-rroot="$3"
-set -- ${cmappings[1]}
-croot="$3"
-sudo e2fsck -y -f /dev/mapper/${rroot?}
-sudo resize2fs /dev/mapper/${rroot?} 800M
-sudo e2image -rap /dev/mapper/${rroot?} /dev/mapper/${croot?}
-
-sudo kpartx -ds raspi3.img
-sudo kpartx -ds compr.img
-
-xz -8 -f compr.img