google suspended app due to "Hockey SDK" which was used long time ago in the code, but doesn't exist there anymore

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> Your app contains content that doesn't comply with the Device and Network Abuse policy. We found youur app is using a non-compliant version of Hockey SDK which contains code to download or install applications from unknown sources outside of Google Play
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> 
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I keep searching through the source codes of android app, but can't find anything related to Hockey SDK. It was removed long time ago.

Any ideas how to debug this issue? Am I missing something?