There is a setting in
/etc/yum.conf that does exactly that :
installonly_limit=. The value of this setting is the number of older kernels that are kept when a new kernel is installed by yum. If the number of installed kernels becomes greater than this, the oldest one gets removed at the same time a new one is installed.
This cleaning can also be done manually with a command that belongs to the yum-utils package :
package-cleanup –oldkernels –count=2