Remove mac from recovery component fieldtrial

We don't appear to be close to ready to experiment with Mac.

Bug: 687231
Change-Id: I4969c762b691faa6c714359116f143afe20d22bf
Reviewed-on: https://chromium-review.googlesource.com/c/1289429Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Robert Kaplow (sloooow) <rkaplow@chromium.org>
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600890}

testing/variations/fieldtrial_testing_config.json

@@ -2151,8 +2151,7 @@
     "ImprovedRecoveryComponent": [
         {
             "platforms": [
-                "windows",
-                "mac"
+                "windows"
             ],
             "experiments": [
                 {