If you check the alipage you will see the two USB-pins you mentioned.
But if you check usb-pinout specs, you will notice that USB has four pins. The two on the soc are the two data pins, the other two pins needed are +5V and GND. So you can feel free to take +5V and GND directly from the power supply/its connector afaik.
Why would you do that?
You already have the device powered up and you can (and should) take the 5V directly from it instead of using a second charger. Or am I misunderstanding something here?
The idea of the old charger is only to unsolder and use the USB connector.
Well, this discussion gets of topics, but interests me. I will open a new one
hmmm, it's been a while, but IIRC USB1 needs pulldowns too GND from Dp/Dm on the host side.
Also for USB2 hardwiring straight to 5V/GND from the powersupply is likely to break chirping - i.e. usb2 speed negotiation.
In specification there is no information about 'USB OTG' port. It is microUSB, I don't like it, USB Type C is much better. Why have they omitted information in specification?