doc(sdk): Events from BackPaginationOutcome are deduplicated…

This patch removes a `TODO` in `BackPaginationOutcome`.
Events it contains are deduplicated by the `EventCache` (see
`event_cache::Deduplicator`) when inserted inside `RoomEventCache`.

Author: Hywan

crates/matrix-sdk/src/event_cache/mod.rs

@@ -671,8 +671,6 @@ pub struct BackPaginationOutcome {
     /// Events are presented in reverse order: the first element of the vec,
     /// if present, is the most "recent" event from the chunk (or
     /// technically, the last one in the topological ordering).
-    ///
-    /// Note: they're not deduplicated (TODO: smart reconciliation).
     pub events: Vec<TimelineEvent>,
 }