Fix deleting task relations

While the request body of a DELETE request *can* have data in it, it is not really supported and pretty much undocumented. As such, it doesn't work in all clients and probably broke with recent updates.
This fixes this by moving all parts of the task relation used to identify it in the url.

More Info: https://stackoverflow.com/a/299696/10924593

pkg/swagger/swagger.json

@@ -5296,7 +5296,9 @@
                         }
                     }
                 }
-            },
+            }
+        },
+        "/tasks/{taskID}/relations/{relationKind}/{otherTaskId}": {
             "delete": {
                 "security": [
                     {
@@ -5329,6 +5331,20 @@
                         "name": "taskID",
                         "in": "path",
                         "required": true
+                    },
+                    {
+                        "type": "string",
+                        "description": "The kind of the relation. See the TaskRelation type for more info.",
+                        "name": "relationKind",
+                        "in": "path",
+                        "required": true
+                    },
+                    {
+                        "type": "integer",
+                        "description": "The id of the other task.",
+                        "name": "otherTaskID",
+                        "in": "path",
+                        "required": true
                     }
                 ],
                 "responses": {