Revert "Add WebView UMA log package name to field trial testing config."

This reverts commit 89f3899b.

Reason for revert: Feature has been launched, remove the fieldtrail testing

Original change's description:
> Add WebView UMA log package name to field trial testing config.
> 
> Bug: 839646
> Change-Id: I0b28b079bd5e44d8f8d9a6057d2740b4f6bc9bcb
> Reviewed-on: https://chromium-review.googlesource.com/c/1370373
> Reviewed-by: Mark Pearson <mpearson@chromium.org>
> Commit-Queue: Tao Bai <michaelbai@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#615807}

TBR=michaelbai@chromium.org,mpearson@chromium.org,holte@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 839646
Change-Id: I029e6e719c509268b8c9aa624b459bc5590b5c3a
Reviewed-on: https://chromium-review.googlesource.com/c/1460445Reviewed-by: Tao Bai <michaelbai@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630631}

testing/variations/fieldtrial_testing_config.json

@@ -5666,21 +5666,6 @@
             ]
         }
     ],
-    "WebViewUmaLogAppPackageName": [
-        {
-            "platforms": [
-                "android_webview"
-            ],
-            "experiments": [
-                {
-                    "name": "WebViewUmaLogAppPackageName",
-                    "enable_features": [
-                        "WebViewUmaLogAppPackageName"
-                    ]
-                }
-            ]
-        }
-    ],
     "WinOOPSelectFileDialog": [
         {
             "platforms": [