I was thinking to use VBUS of USB as power supply for my circuit. I have made previously power supply like that using LC filter. However it picked 50Hz buzz, so my teacher came up with an idea to get rid of ground loop and cut off ground and shielding of usb cable.
You should ask your teacher why the shield would cause a problem, but the power supply gnd connection would not. And if your teacher really meant cut off
ground and shielding as you wrote then ask how would the power supply current return to the power supply with only the one +5V power supply wire?
You should also ask where the "loop" occurs in a microphone, since the audio signal shield and the USB cable shield are presumably connected together at the computer, and the input is acoustic, there are no other devices attached to form a loop.
A slightly less sarcastic answer: it is possible that if the USB port used is on the opposite side of the computer from the line input, that potential differences across the computer chassis could cause current to flow through the USB shield, through the microphone, and then through the audio cable shield to complete the current loop across the computer motherboard. That should be much less of a concern if the USB connector and audio connector are on the same side of the computer, since there should not be much if any potential difference between connectors in close physical proximity.
One of the advantages of properly designed balanced connections is that the shield is not used as one of the audio signal conductors as it is on single ended connections, so shield currents will have very little effect on the audio signal.
When you do get a microphone preamp with phantom power and balanced connections it is relatively simple to create a circuit powered by phantom power, no more complicated than what you are already building.
See this page for one of the regular group posters who has published a lot of information, including how you can build up the circuit on strip-board.
https://www.opic.jp137.com/
That page has a few different variants, including for capsules which need external polarization voltage or have dual diaphragms.
For an electret capsule like you have this is the relevant project:
https://www.opic.jp137.com/index-fet.html
Click the schematic picture at the top of that page to get a PDF file with more detail.