[ios] Disable UKM tests everywhere on iOS.

Previous CL was iPad only, but these tests are failing all over.

TBR=olivierrobin@chromium.org

Bug: 853992
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I777f2677202564eb26a8b2735f3df55a1dfff3d9
Reviewed-on: https://chromium-review.googlesource.com/1106220Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568470}

ios/chrome/browser/metrics/ukm_egtest.mm

@@ -254,9 +254,7 @@ void SignOut() {
 
 // Per crbug.com/853992, Entire test suite is failing regularly.
 + (NSArray*)testInvocations {
-  if (IsIPadIdiom())
-    return @[];
-  return [super testInvocations];
+  return @[];
 }
 
 + (void)setUp {