How to fully unroot + recover device into factory default

I've rooted with King Root. Since I have got troubles with GPS and need to send phone for repair, I need to completely unroot with no trace that phone was rooted. Do you know hot to do it...? I've found that after unroot via King root RW is still mentioned while booting, so it's not clean unroot... Please explain if you can step by step.

Thank you very much.

Martin

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