Remove FIXME in member.rs

This is being adressed in the spec, see
https://github.com/matrix-org/matrix-doc/pull/2591

src/room/member.rs

@@ -57,8 +57,6 @@ ruma_event! {
             /// contain information about that invitation.
             #[serde(skip_serializing_if = "Option::is_none")]
             pub third_party_invite: Option<ThirdPartyInvite>,
-
-            // FIXME: spec says there is an unsigned field in content, which seems like an error.
         },
     }
 }