Drop Debian SoftHSM v2.4 CI jobs
The base image is in the process of being upgraded from Debian Buster to Debian Bullseye, which has SoftHSM v2.6, the same SoftHSM version we already test PKCS#11 with on Fedora. We don't need to test with two SoftHSM 2.6 versions, drop CI jobs running on the base image.
Prereq for: isc-projects/images!145