On my Samsung Galaxy C5 Pro, running Android 7.0 (Hong Kong ROM), I was able to delete the Samsung built-in calendar "My Calendars" by installing another calendar called
Business Calendar 2 as follows:
Hamburger menu
Calendars & Task lists
Tap pen (edit) icon for offending calendar
Tap bin icon to completely delete
Click 'Yes' at the confirmation prompt
Done!
I have no idea why Google's 'My Calendar' is displayed as 'My
calendar
s', only a big deal if you have OCD which I happen to have a little bit.