The "registration" text can be changed in the configuration under the tab language by editing your hikashop translation file. The registration string should be near the end of the file.
If you want to remove the login form on the checkout, you can edit the CSS under the tab display of the configuration and change the code
.hikashop_checkout_login_left_part{
width:250px;
float:left;
}
.hikashop_checkout_login_right_part{
width:500px;
float:left;
}
to
.hikashop_checkout_login_left_part{
display:none;
}
If you want just to remove the top fields password, username and name of the registration form, you should activate the option "simplified registration" in the configuration under the tab display.