When to get rid of undergraduate stuff on CV
I do not think there can be a generalised answer to your question, however there are multiple factors to consider about what you might want to put on your CV. 


As Solar Mike mentioned in the comments some programmes have certain requirements. These should obviously not be omitted.
Some programmes might have preferences about their applicants, e.g. valuing research experience, volunteer work, social engagement or experience abroad. Such preferences might become obvious from the descriptions on their website, advertisements or alumni interviews. You could try to tune your CV according to these. 
You can also choose how you want to portray yourself. Maybe you want to show extra enthusiasm by including that besides coursework and research you have even spent summers doing internships? Or you want to focus on just the hard facts?
Which leads a potentially very important point: The length of your CV. If you think summer internships do not add anymore weight to your research experience and rather obstruct the reader from getting a quick overview of your education and work experience, then there is little point in keeping it on there.