fix(sdk)!: Remove SyncEvent implementation for InitialStateEvent

Initial state events aren't actually received through sync, they're sent from
the client to the server when creating a room.

This change makes it impossible to register event handlers for initial state
events. Previously, this was allowed, but the event handler was never called.

crates/matrix-sdk/src/event_handler.rs

@@ -77,7 +77,6 @@ pub enum EventKind {
     OriginalState,
     RedactedState,
     StrippedState,
-    InitialState,
     ToDevice,
     Presence,
 }
@@ -681,14 +680,6 @@ mod static_events {
         const TYPE: &'static str = C::TYPE;
     }
 
-    impl<C> SyncEvent for events::InitialStateEvent<C>
-    where
-        C: StaticEventContent + StateEventContent,
-    {
-        const KIND: EventKind = EventKind::InitialState;
-        const TYPE: &'static str = C::TYPE;
-    }
-
     impl<C> SyncEvent for events::ToDeviceEvent<C>
     where
         C: StaticEventContent + ToDeviceEventContent,