Similar issue has been reported by one of our customer, below is the resolution.
Resolution :
As of now, you need to remove the old workflow versions as Restconnector is present in old versions.
After deleting the old workflow versions, you can try to delete the Restconnector.
We will implement the below request in the next release:
Regarding the original workflow name which used the REST connector should be displayed while deleting the REST connector, the issue will be fixed in the next release.
A user looks for a feature to delete all versions of the workflow at one time instead of deleting them one by one.