Removing the project that holds the Insights configuration raises an error

Steps to reproduce:

  1. Select a project that holds the Insights configuration in the group settings: Screen_Shot_2019-04-10_at_19.42.45
  2. Save the group settings
  3. Clear the selected project: Screen_Shot_2019-04-10_at_19.42.51
  4. Save the group settings
  5. A not-null constraint error is raised: Screen_Shot_2019-04-10_at_19.43.02

I think the insights record should be deleted instead of only trying to clear its namespace_id column.

/cc @godfat