Remove $schema keys in JSON schemas

They are only in a few schemas and use the wrong version of the spec

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

data/event-schemas/schema/core-event-schema/msgtype_infos/image_info.yaml

@@ -1,4 +1,3 @@
-$schema: http://json-schema.org/draft-04/schema#
 description: Metadata about an image.
 properties:
   h:

data/event-schemas/schema/core-event-schema/msgtype_infos/thumbnail_info.yaml

@@ -1,4 +1,3 @@
-$schema: http://json-schema.org/draft-04/schema#
 description: Metadata about a thumbnail image.
 properties:
   h:

data/event-schemas/schema/m.room.third_party_invite.yaml

@@ -1,5 +1,4 @@
 ---
-$schema: http://json-schema.org/draft-04/schema#
 allOf:
   - $ref: core-event-schema/state_event.yaml
 description: "Acts as an `m.room.member` invite event, where there isn't a target user_id to invite. This event contains a token and a public key whose private key must be used to sign the token. Any user who can present that signature may use this invitation to join the target room."