update-initramfs leaves old initrd images around in
/boot, even when the kernel package is deinstalled. Deleting the file does not help, the next run of
update-initramfs will recreate the file.
The solution is simple: all kernels which need to be rebuild are in
/var/lib/initramfs-tools/ - just delete the old ones, and the images in
/boot, and they will not be recreated.