[CentOS] How do I remove a kernel

Tue Jan 8 22:22:27 UTC 2019
Robert Moskowitz <rgm at htt-consult.com>

I have 4 kernels in /boot, leaving on 20MB which is not enough for the 
next one.

I had installonly_limit= set at 5, as there were some kernel problems.  
After I got the error that there was not enough room for another kernel, 
I set installonly_limit= to 3 and did the update with --exclude=kernel*

That worked to update everything else, but not remove the oldest kernel.

How can I remove the oldest kernel to make room for the new one?