The only purpose that an activation key serves is allowing updates. The presence of or lack thereof will not alter the functionality of the plugin.
As far as defining the license key using a define( 'ACF_PRO_LICENSE', '[key]' );, this would only be relevant if you are going to have your theme install ACF Pro like any other plugin. I believe that doing this would be against the ToU Rules.
- ACF would no longer be part of your theme
- You would be sharing license key information
Your only real choices are to include it in the theme and provide ACF updates via whatever method you plan on to provide your premium theme updates or require that the person that purchases your theme also purchases a license for ACF Pro.
I could be wrong about this, I’m just another user and that is my interpretation of the rules. You might want to contact the developers by submitting a ticket in your account to verify my interpretation.
