Revert "Add MacMDDownloadShelf to fieldtrial_testing_config.json"

This reverts commit 22e3eede.

Reason for revert: This change looks causing failures on Mac ASan 64 bot.

Original change's description:
> Add MacMDDownloadShelf to fieldtrial_testing_config.json
> 
> Change-Id: I00799bf180fc94b3a8b30e0a77bdc7473cf58ca4
> Bug: 589943
> Reviewed-on: https://chromium-review.googlesource.com/887385
> Reviewed-by: Ilya Sherman <isherman@chromium.org>
> Commit-Queue: Sidney San Martín <sdy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#532044}

TBR=isherman@chromium.org,sdy@chromium.org

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

Bug: 589943
Change-Id: Id1a2fbef9c06dcd035e30974ab0da18cb9021377
Reviewed-on: https://chromium-review.googlesource.com/890840Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532332}

testing/variations/fieldtrial_testing_config.json

@@ -1855,21 +1855,6 @@
             ]
         }
     ],
-    "MacMDDownloadShelf": [
-        {
-            "platforms": [
-                "mac"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "MacMDDownloadShelf"
-                    ]
-                }
-            ]
-        }
-    ],
     "MacV2Sandbox": [
         {
             "platforms": [