Remove kernel version as proxy requirement for OCSP test

[dtrudg]

In e2e/verify/verify.go we require a kernel >= 4.18 to start an OCSP responder, and run the OCSP tests.

This kernel version requirement was used as a proxy for CentOS 7 / SLES 12... which don't support some algorithms in the OCSP chain.

We can now remove the kernel version requirement, as we won't support these distributions for 4.2.