how to remove country code from incoming calls and messages

Try going to Settings>Phone>Dial Assist and turning it on.

Apple isn't adding it. That's what's coming from caller ID. Caller ID is a carrier feature.

you cannot do it and also it does not affect you in any way when calling or texting in fact when adding a new number directly with the country code and you may be in a different region it is there without the need of modification