You can hide that with the help of CSS as an approach.
Before that you need to read this article, In that David has explained how the CSS class is being referred by each of the overlay calendar.
From that I have found that if you place below CSS snippet as a script editor web part on that page or you can place it in your own CSS file.
<style type="text/css">
.ms-acal-apanel-color { display: none; }
</style>
Make sure this CSS snippet only appears where you want to hide the main calendar item as shown in your snap.
In my example there were two calendars:

As I have inspected on HTML DOM elements found the following:

In above snap you can see in both li
elements different class is provided: ms-acal-apanel-color
(default calendar color) and ms-acal-apanel-color4
(other overlay calendar color).
I have just applied CSS to hide the first element of that calendar items.