Hi,
The name field is for the user account in Joomla. That's the name displayed by Joomla in the login module once you're logged in for example.
The firstname and lastname fields are for the address of the customer.
When you're in "simplified" registration mode, HikaShop automatically uses the firstname and lastname fields to fill the "name" of the Joomla user account.
When you're in normal registration mode, HikaShop displays the same fields as the Joomla registration form, plus the address fields. That's why you see that "name" field there.
To remove it, go in the Display>Views menu, edit the sub_block_login_registration view file and use the view builder to delete the name area.