If you would like to remove the First and Last name input fields from the Identity Management registration form, you should delete or comment out the following lines in the HTML code of the Piano ID Register Page template:
HTML
<p class="input-group with-icon">
<input fieldRegisterFirstName type="text" value="">
<span class="placeholder"><i class="icon icon-user"></i><t>first name</t></span>
<span showIfFirstNameInvalid class="error-message"><t>First name is required</t></span>
<span showIfFirstNameInvalidBy="SERVER_ERROR" class="error-message" setFirstNameServerErrorMessage></span>
</p>
<p class="input-group with-icon">
<input fieldRegisterLastName type="text" value="">
<span class="placeholder"><i class="icon icon-user"></i><t>last name</t></span>
<span showIfForLastNameInvalid class="error-message"><t>Last name is required</t></span>
<span showIfLastNameInvalidBy="SERVER_ERROR" class="error-message" setLastNameServerErrorMessage></span>
</p>
Please note, that you should make sure, that the first and last names are not required in your Identity Management settings as described here.