typing status: Cut redundant is-this-PMs condition.

When this condition is true, user_ids_array will always be `undefined`
and so we won't reach this conditional anyway.

static/js/typing.js

@@ -48,15 +48,6 @@ function is_valid_conversation(user_ids_array) {
         return false;
     }
 
-    if (compose_state.get_message_type() !== 'private') {
-        // We only use typing indicators in PMs for now.
-        // There was originally some support for having
-        // typing indicators related to stream conversations,
-        // but the initial rollout led to users being
-        // confused by them.  We may revisit this.
-        return false;
-    }
-
     return true;
 }