ffi: remove the Reaction::count field

It's exactly the same as `Reaction::senders`'s length.

bindings/matrix-sdk-ffi/src/timeline/content.rs

@@ -269,7 +269,6 @@ impl EncryptedMessage {
 #[derive(Clone, uniffi::Record)]
 pub struct Reaction {
     pub key: String,
-    pub count: u64,
     pub senders: Vec<ReactionSenderData>,
 }
 

bindings/matrix-sdk-ffi/src/timeline/mod.rs

@@ -978,7 +978,6 @@ impl EventTimelineItem {
             .iter()
             .map(|(k, v)| Reaction {
                 key: k.to_owned(),
-                count: v.len().try_into().unwrap(),
                 senders: v
                     .senders()
                     .map(|v| ReactionSenderData {