Remove vestigial SAML references

[brodygov]

The dashboard switched from SAML to OIDC a year ago, but still has a lot of references to SAML in disabled tests and in dependencies.

https://github.com/18F/identity-dashboard/commit/bac2d0e5d4bd36c08512e5ca0a2cb59ffc42c340

We should ideally replace these tests with relevant tests for OIDC and remove all the unused references to SAML.

There are a number of config variables that refer to SAML when they are actually used for OIDC.

• saml_sp_private_key is actually used for OIDC
• saml_sp_certificate is not used, since OIDC doesn't use certificates, but it's required by config/initializers/figaro.rb