nix: Remove ability to build pindakaas SD image

The idea to create an image burnable on a SD card that is already
configured is a nice one, however I encountered many issues:

- Used something that was designed to be an installer:
    - Cannot change from unencrypted ext4 partition (AFAICS)
    - Comes with installer tools (i.e. tools for various filesystems),
      which creates conflicts
- Takes a lot of space and time to build, had to put /tmp on a non-tmpfs
- Has to extend the filesystem manually afterwards
- I think it rebuilt the full system on nixos-rebuilt anyways. Maybe
  because I used binfmt compilation? Could also be because installer has
  some overlays.

Since I had nix on Manjaro already, I surely could have built the
system directly on the SD card from there. Maybe that's what I will do
to test the partitionning/boot scheme setup actually.

config/nix/build-pindakaas-image.sh

@@ -1,22 +0,0 @@
-#/usr/bin/env sh
-
-# Build a disk image for testing on pindakaas via sd card
-
-if ! command -v nom-build
-then
-    exec nix-shell -p nix-output-monitor --run $0
-fi
-
-extra=""
-if [ "$(uname -m)" != "aarch64" ]
-then
-    if [ ! -f /proc/sys/fs/binfmt_misc/qemu-aarch64 ]
-    then
-        echo "Use binfmt QEMU!"
-        echo "https://nixos.wiki/wiki/NixOS_on_ARM#Compiling_through_binfmt_QEMU"
-        exit 1
-    fi
-    extra="--argstr system aarch64-linux"
-fi
-nom-build '<nixpkgs/nixos>' -A config.system.build.sdImage \
-    -I nixos-config=./pindakaas.nix $extra --show-trace

config/nix/pindakaas.nix

@@ -2,11 +2,9 @@
 {
   imports = [
     <nixos-hardware/pine64/pinebook-pro>
-    <nixpkgs/nixos/modules/installer/sd-card/sd-image-aarch64.nix>
     ./os/loader.nix
   ];
   # nixos-hardware use latest kernel by default. This obviously runs quickly out of sync with zfs packages.
-  # zfs packages are included despite me asking anything zfs, probably they're in the image for installation purposes?
   boot.kernelPackages = config.boot.zfs.package.latestCompatibleLinuxPackages;
 
   networking.hostName = "pindakaas";