Can any body tell how to remove it completely so that it is not hearable?
As suggested already, the signal has harmonics.
You can diminish it further by keep filtering the rest of the peaks of the spectrum with a notch filter, or using the noise reduction option in Audacity.
The latter option is better when the interference is over a useful part of the recording. To apply that filter, select a part of the recording that is representative of the noise profile you are dealing with. Audacity will use this to extract the noise spectrum and then create a filter that cancels it. You can then take that profile and apply it to other points in the recording to reduce its impact.
It will be bonus for me if someone can tell why this high pitch noise occur in audio?
It sounds like interference and it is likely that it is picked up as a square wave. It definitely doesn't sound like a hum, more like a high pitch pipe organ sound with two or three components.
It might be coming from unshielded cables, strong sources nearby an amplifier (e.g. WiFi router / dongle, LED lightstrip, TV, other) or if it is an audio USB dongle, of dubious built quality, it might be picking up interference from the data exchange going on on the bus. It's hard to tell without knowing what is this a recording of (?).
Hope this helps.
