In this post at Android Central, the suggestion is:

1. Open a root explorer, go to /system/media/audio/ui
2. Rename PowerOn.ogg → PowerOn.ogg.disabled, PowerOff.ogg → PowerOff.ogg.disabled

This requires root.

^{This seems promising, but in my HTC E9 phone (Android 5) these files don't exist :(}

In this post at Android Central, the suggestion is:

1. Open a root explorer, go to /system/media/audio/ui
2. Rename PowerOn.ogg to PowerOn.ogg.disabled, PowerOff.ogg to PowerOff.ogg.disabled

This requires root.

^{This seems promising, but in my HTC E9 phone (Android 5) these files don't exist :(}