it is not possible to uninstall RT kernel

Even after I do apt-get purge it is still there

$ sudo apt-get purge linux-image-5.15.74-rt53-raspi
$ sudo reboot
$ apt list --installed | grep linux-image
linux-image-5.15.0-1027-raspi/jammy-updates,jammy-security,now 5.15.0-1027.29 arm64 [installed,automatic]
linux-image-raspi/jammy-updates,jammy-security,now 5.15.0.1027.24 arm64 [installed]

$ uname -a
root@ubuntu:/home/ubuntu# uname -a
Linux ubuntu 5.15.74-rt53-raspi #1 SMP PREEMPT_RT Mon Jan 16 13:25:26 UTC 2023 aarch64 aarch64 aarch64 GNU/Linux

This is because /boot/firmware/vmlinuz is still RT kernel

$ ls -la /boot/firmware/vmlinuz*
-rwxr-xr-x 1 root root 10320014 May  6 11:03 /boot/firmware/vmlinuz
-rwxr-xr-x 1 root root 10338087 May  6 10:57 /boot/firmware/vmlinuz.bak

$ cp /boot/vmlinuz-5.15.0-1027-raspi /boot/firmware/vmlinuz
$ cp /boot/initrd.img-5.15.0-1027-raspi /boot/firmware/initrd.img
$ sudo reboot
$ uname -a
Linux ubuntu 5.15.0-1027-raspi #29-Ubuntu SMP PREEMPT Mon Apr 3 10:12:21 UTC 2023 aarch64 aarch64 aarch64 GNU/Linux

even after that

ubuntu@ubuntu:~$ apt list --installed | grep rt53
linux-libc-dev/now 5.15.74-rt53-raspi-1 arm64 [installed,local]

ubuntu@ubuntu:~$ sudo apt-get install --reinstall linux-libc-dev
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Reinstallation of linux-libc-dev is not possible, it cannot be downloaded.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

$ wget http://ports.ubuntu.com/pool/main/l/linux/linux-libc-dev_5.15.0-25.25_arm64.deb
$ sudo dpkg -i linux-libc-dev_5.15.0-25.25_arm64.deb
$ apt list --installed | grep linux-libc
$ linux-libc-dev/jammy,now 5.15.0-25.25 arm64 [installed,upgradable to: 5.15.0-71.78]

ros-realtime / ros-realtime-rpi4-image

it is not possible to uninstall RT kernel #63