chore(sdk): Remove invalid comment

I have tried for hours and have concluded it is probably not possible
even with GATs to have RawEvent borrow its inner value.
It is also not a commonly-used feature, so removing the unnecessary
clones is not that important.

crates/matrix-sdk/src/event_handler/context.rs

@@ -58,8 +58,6 @@ impl EventHandlerContext for room::Room {
 ///
 /// Used as a context argument for event handlers (see
 /// [`Client::add_event_handler`]).
-// FIXME: This could be made to not own the raw JSON value with some changes to
-//        the traits above, but only with GATs.
 #[derive(Clone, Debug)]
 pub struct RawEvent(Box<RawJsonValue>);