Limiting the testing of the UmaAndUkmDemographics study to iOS.

Don't test the other platforms because they now enable the
experimented features by default.

Change-Id: I08fb048e73d4508a4e9155cb1d5e6307db83c47e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095370
Commit-Queue: Vincent Boisselle <vincb@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749256}

testing/variations/fieldtrial_testing_config.json

@@ -5805,11 +5805,6 @@
     "UmaAndUkmDemographics": [
         {
             "platforms": [
-                "android",
-                "chromeos",
-                "linux",
-                "mac",
-                "windows",
                 "ios"
             ],
             "experiments": [