Remove all client code that checks for X.509 certificates

Although all dqsegdb client code will use a SciToken, if one is available, and X.509 certificates are no longer being generated, so the X.509 code has no effect (other than producing deprecation warnings), the code that checks for X.509 certificates should be removed (perhaps especially to get rid of the deprecation warnings).

Some code locations:

• in urifunctions.py, import of find_x509_credentials, # also try and find an X.509 credential block in _auth_request()
• (maybe only in that one file?)