So normally you remove kernels using YaST but that does not work if you did a manual compile outside of YaST. You failed to mention your openSUSE version used. To delete a manually compiled kernel you would remove the folder you did this in and these two others:
Delete the /lib/module/version-desktop folder and the kernel modules: 1.7 GB (Must be root to remove)
Delete three files in the /boot folder: 21 MB, called initrd-version, System.map-version & vmlunuz-version (Must Be root to remove)
I have a blog on compiling kernels you can use and include links to three bash scripts I have written to compile, remove and fetch any kernel version, past or present.