Actually on the JIRA REST API documentation (https://docs.atlassian.com/jira/REST/latest/#api/2/project/

/role-deleteActor) there's no example to remove a user/group from a project role:

DELETE
Remove actors from a project role.

available response representations:

204 [expand]
Returned if the actor was successfully removed from the project role.

404 [expand]
Returned if the project or role is not found, the calling user does not have permission to view it, or does not have permission to modify the actors in the project role.

It may lead a customer (and I admit, myself) to use the example code from the POST/PUT methods on the DELETE method, and it won't work. The DELETE method must be invoked with the following URL:

• http://JIRA_URL:8080/rest/api/2/project/{PROJECTKEY}/role/{ROLEID}?user={username}
  or
• http://JIRA_URL:8080/rest/api/2/project/{PROJECTKEY}/role/{ROLEID}?group={groupname}