I don't find any way to set, at site level, that any new added activity to my moodle comes with "grade type" filled as "none" by default.

Perhaps someone can consider that an activity has to be graded, but in our case of MOOC and H5P activities, actually it is not true.

I suggest to include "grade type" setting on:

• Administration > Grades > General settings
  OR 
• Administration > Activity Modules > Common activity settings