Revert part of ag/6685504

We don't actually need to post a message here because the same
message was scheduled to resume the top activities for that case
in ActivityStartController#startHomeActivity().

Bug: 131401344
Bug: 126747441
Test: MultiDisplayPolicyTests
Test: DeviceOwnerTest

Change-Id: I347134eb4246eccf308c658bd71109ba969cb78c