Removing assignee is uninformative
### Problem
I cannot see which assignee I removed.

### Usecase
I accidentally removed the (single) assignee from an issue.. the system note didn't help me out.
### Proposal
Should state which assignee is unassigned :wink:
### Design



#### CC/Call to actions
@victorwu @smcgivern (assigned this gitlab-ce~2311884 , but can almost be considered a bug I think)
@ClemMakesApps (as you implemented the multiple assignee feature)
issue