iOS: Don't run modules_unittests on iOS simulator

This is the only failing test of the currently deployed
at the iOS Simulator bots. Let's disable it so we can promote
the passing tests to the main waterfall and the trybots.

BUG=4755
TBR=henrika@webrtc.org

Review URL: https://codereview.webrtc.org/1722523002 .

Cr-Commit-Position: refs/heads/master@{#11703}

webrtc/build/ios/tests/common_tests.json

@@ -12,9 +12,6 @@
     {
       "app": "modules_tests"
     },
-    {
-      "app": "modules_unittests"
-    },
     {
       "app": "rtc_api_objc_tests"
     },