Remove cloud-initramfs-rooturl to fix boot failure

Conflicts with custom netboot script that provides essential tmpfs overlay.
cloud-initramfs-rooturl mounts squashfs read-only, but diskless nodes need
writable overlay for /var/log, /tmp, /etc, and runtime state.

Fixes: Can't open blockdev error during boot

build-image.sh

@@ -61,7 +61,6 @@ apt-get upgrade -y
 apt-get install -y \
     linux-image-generic \
     linux-firmware \
-    cloud-initramfs-rooturl \
     busybox-initramfs \
     initramfs-tools \
     keyboard-configuration \