How to properly get rid of previous version of linux kernels?

[EOS]

Did you try this:

Too many grub entries

I would try enabling the submenu on grub, which I believe is disabled by default in EOS. Back up your system. Sudo edit /etc/default/grub, comment out the line GRUB_DISABLE_SUBMENU=y, and then run sudo grub-mkconfig -o /boot/grub/grub.cfg to rebuild grub. I believe that will group the various kernels for each install under submenus.