Remove USB image support (for Endless Key V1)

The original incarnation of Endless Key was a hybrid USB stick that
contained some Kolibri stuff and a Kiwix encyclopedia usable on Windows,
but also a bootable Endless OS image that could access the same content.

This is a thing of the past and its presence is confusing when we have
another thing, also called Endless Key, which can be preloaded in OS
images.

It has also become a practical problem because exfat-utils was removed
from Debian Bookworm so image builds on that branch were broken in order
to support a mode we no longer use.

Remove it all.

https://phabricator.endlessm.com/T35093

config/defaults.ini

@@ -66,8 +66,6 @@ packages_add =
   endless-ca-cert
   eos-keyring
   eos-tech-support
-  exfat-fuse
-  exfat-utils
   fdisk
   flatpak
   gir1.2-flatpak-1.0
@@ -152,9 +150,6 @@ boot_zip = true
 # Don't build ISOs by default
 iso = false
 
-# Don't build USB images by default
-usb = false
-
 # Don't build VM images by default
 # This setting only works for arch=amd64, for all other values this is a no-op
 # (please also add the comment above to anywhere 'vm_image' is changed)
@@ -308,10 +303,6 @@ kolibri_pkgspec = https://github.com/learningequality/kolibri/releases/download/
 # (ex. artist, explorer, spanish etc).
 collections =
 
-[usb]
-size = 16000000000
-free_space = 1000
-
 [ostree]
 # Repository setup. By default the remote, remote repo and OS name are
 # all named eos.

stages/eib_image

@@ -160,12 +160,6 @@ dpkgdir="${OSTREE_DEPLOYMENT}"/var/lib/dpkg
 packageinfo="${EIB_TMPDIR}"/packages.txt
 dpkg-query -W --admindir="${dpkgdir}" > "${packageinfo}"
 
-# Create a clean USB image workspace directory
-if [[ "${EIB_IMAGE_USB}" = "true" ]]; then
-  export EIB_USB_IMAGE_WORKSPACE_DIR="${EIB_TMPDIR}/usb-image-extra"
-  recreate_dir "${EIB_USB_IMAGE_WORKSPACE_DIR}"
-fi
-
 # Add package info to manifest using helper. This needs to be run here
 # rather than in an image hook because the dpkg database will be masked
 # by the OS /var mount.
@@ -626,14 +620,6 @@ EOF
     "${EIB_HELPERSDIR}"/create-vm-image "$(eib_outfile ovf.zip)" "${img}"
   fi
 
-  # Create USB image if required
-  # This expects to have an ISO image for eos-write-live-image.
-  if [[ "${EIB_IMAGE_USB}" = "true" && "${EIB_IMAGE_ISO}" = "true" && "${EIB_IMAGE_BOOT_ZIP}" = "true" ]]; then
-    "${EIB_HELPERSDIR}"/create-usb-image "${version}" "$(eib_outfile usb.img)" \
-      "$(eib_outfile iso)" "${boot_zip}" "${img_csum}" "${boot_zip}.sha256" \
-      "${img_asc}" "${boot_zip}.asc"
-  fi
-
   # Await signing and checksumming subprocesses
   wait