Remove unused images, shift to Mermaid

If merged, this work will replace the second of two icky old images
on this page with Mermaid graphs that we can actually REVISE.

Also deletes the unneeded images.

doc/user/project/code_owners.md

@@ -83,7 +83,13 @@ The eligible Code Owners are:
 You can [invite](members/share_project_with_groups.md) **Subgroup Y** to **Project A**
 so that their members also become eligible Code Owners.
 
-![Invite subgroup members to become eligible Code Owners](img/code_owners_invite_members_v13_4.png)
+```mermaid
+graph LR
+    A[Parent group X] -->|owns| B[Project A]
+    A -->|contains| C[Subgroup Y]
+    C -.->D{Invite to<br/>Project A?} -.->|Approvals can be<br/> required or optional| B
+    C -.->E{Add as Code Owners<br/>to Project A?} -.->|Approvals can only<br/>be optional| B
+```
 
 If you do not invite **Subgroup Y** to **Project A**, but make them Code Owners, their approval
 of the merge request becomes optional.