If you set “I sell to only one country” in WooCommerce settings, that country will appear in checkout and the country field will be readonly.
You can hide it with some custom css:
#billing_country_field {
display: none;
}
Custom CSS can be entered at:
Dashboard > Appearance > Customize > Additional CSS
If you have any PHP code snippets relating to this I suggest you remove them.
Even though now the country is hidden, it must still be there in the markup so the form handler knows the country and so is able to complete orders.
I think it does serve a purpose if users can see it. The nature of the internet is that users may come from any country. They may miss your notice that says “We only sell to — country”. When they get to checkout, and can’t alter the country, they will realise you serve one country only. You will not want to get orders from addresses which are in another country.