Clean up unused PKI test files.

Previously these files were being used to integration testing of mutual
TLS authentication. They're not any longer and can be removed.

.secrets.baseline

@@ -3,7 +3,7 @@
     "files": "^.secrets.baseline$|^.*pgsslrootcert.yml$",
     "lines": null
   },
-  "generated_at": "2019-12-18T22:26:52Z",
+  "generated_at": "2020-01-09T11:35:03Z",
   "plugins_used": [
     {
       "base64_limit": 4.5,
@@ -111,15 +111,6 @@
         "type": "Secret Keyword"
       }
     ],
-    "ssl/certificate-authority/ca.key": [
-      {
-        "hashed_secret": "be4fc4886bd949b369d5e092eb87494f12e57e5b",
-        "is_secret": false,
-        "is_verified": false,
-        "line_number": 1,
-        "type": "Private Key"
-      }
-    ],
     "ssl/client-certs/atat.mil.key": [
       {
         "hashed_secret": "be4fc4886bd949b369d5e092eb87494f12e57e5b",