OpenSSL no longer required for web-platform-tests

Previously OpenSSL was required to generate certificates for 
web-platform tests. I just landed a PR [1] to use certificates checked 
in to the repository instead (this is the same approach gecko uses). 
Therefore OpenSSL is no longer required to run tests requiring SSL.

Hopefully this makes running wpt easier, particularly on Windows. Note 
that certutil *is* still required to install the cetificates into the 
Firefox certdb.

[1] https://github.com/w3c/web-platform-tests/pull/6938