Remove "roles" column in the Portfolio "members" table in django admin

[abroddrick]

Issue description

Right now on the portfolio page in django admin, we show the users in the portfolio with admins in one section and members in a section below. The member section has a role column, however, we are changing permissions and how we communicate them to users so the roles we currently display on admin are no longer applicable. We also are not going to prioritize determining replacement human readable phrases anytime soon. We should just remove the code and column as it no longer applies. (we can always look in git commit history if we want to see the code again)

Acceptance criteria

• remove the roles column from the django admin portfolio members table
• delete the unused code and make appropriate changes to clean up this area

Additional context

No response

Links to other issues

No response