Remove old Linux kernel
Check the /lib/modules folder to see a full list of old kernels installed or previously installed now removed.
Normally distributions will keep at least 2 kernels in case the new kernel doesn't work well, you have a choice to roll back.
Debian or Ubuntu
Remove 2.6.24-21-generic, APT will remove the packages and regenerate GRUB menu.lst.
RHEL and OEL
Check folder /lib/modules for a list of installed kernels.
Get a list of packages, remove it by using yum.
Note: The corresponding folder in /lib/modules will not be removed by package management tool.