hide profile title in Fieldset (legend)
The simplest way is to use CSS to have something like the following

.whatevercontext fieldset.crm-profile-id-13 legend {
  display: none
}


and then put that into your site's theme CSS file alongside other customizations you do for your site.  Alternatively, you could create a special CSS file and enter the URL in Administer > System Settings > Resource URLs.  Both methods avoid hacking CiviCRM's CSS code.

However, you should double-check the affected form(s) after each upgrade just to make sure the selector hasn't changed.