Disable SpeculativePreconnectValidation field trial

This CL temporarily disables the SpeculativePreconnectValidation field
trial in order to assess its impact on the perf dashboard. This feature
is planned to be turned on by default after the performance evaluation.

Bug: 808036
Change-Id: I69de5a26eba86b8e4cad11c4cf534c3d13e73894
Reviewed-on: https://chromium-review.googlesource.com/1028052Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553955}

testing/variations/fieldtrial_testing_config.json

@@ -3633,27 +3633,6 @@
             ]
         }
     ],
-    "SpeculativePreconnectValidation": [
-        {
-            "platforms": [
-                "win",
-                "mac",
-                "linux",
-                "android"
-            ],
-            "experiments": [
-                {
-                    "name": "Preconnect2",
-                    "params": {
-                        "mode": "preconnect"
-                    },
-                    "enable_features": [
-                        "SpeculativePreconnect"
-                    ]
-                }
-            ]
-        }
-    ],
     "StabilityDebugging": [
         {
             "platforms": [