Remove definition of Event for `/notifications` (#3525)

The documentation for the `/notifications` API had its own special definition
of what an Event was, which was used nowhere else.

The common definition isn't perfect, but it *is* common, so it gives us a
better starting place for improvement.

data/api/client-server/definitions/event.yaml

@@ -1,65 +0,0 @@
-# Copyright 2016 OpenMarket Ltd
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-properties:
-  event_id:
-    description: The ID of this event, if applicable.
-    type: string
-  content:
-    description: The content of this event. The fields in this object will vary depending
-      on the type of event.
-    title: EventContent
-    type: object
-  origin_server_ts:
-    description: Timestamp in milliseconds on originating homeserver when this event
-      was sent.
-    format: int64
-    type: integer
-  sender:
-    description: The MXID of the user who sent this event.
-    type: string
-  state_key:
-    description: Optional. This key will only be present for state events. A unique
-      key which defines the overwriting semantics for this piece of room state.
-    type: string
-  type:
-    description: The type of event.
-    type: string
-  unsigned:
-    description: Information about this event which was not sent by the originating
-      homeserver
-    properties:
-      age:
-        description: Time in milliseconds since the event was sent.
-        format: int64
-        type: integer
-      prev_content:
-        description: Optional. The previous `content` for this state. This will
-          be present only for state events appearing in the `timeline`. If this
-          is not a state event, or there is no previous content, this key will be
-          missing.
-        title: EventContent
-        type: object
-      transaction_id:
-        description: Optional. The transaction ID set when this message was sent.
-          This key will only be present for message events sent by the device calling
-          this API.
-        type: string
-      redacted_because:
-        description: Optional. The event that redacted this event, if any.
-        title: Event
-        type: object
-    title: UnsignedData
-    type: object
-title: Event
-type: object

data/api/client-server/notifications.yaml

@@ -113,7 +113,7 @@ paths:
                       title: Event
                       description: The Event object for the event that triggered the notification.
                       allOf:
-                        - "$ref": "definitions/event.yaml"
+                        - $ref: ../../event-schemas/schema/core-event-schema/sync_room_event.yaml
                     profile_tag:
                       type: string
                       description: The profile tag of the rule that matched this event.