[geary] Fix a warning in ConversationSet when removing a conversation

[Michael Gratton <mjog src gnome org>]

commit 77c66735cd0f788f3adf6cc3ead8ba007754dff5
Author: Michael Gratton <mike vee net>
Date:   Sun Feb 17 22:15:12 2019 +1100

    Fix a warning in ConversationSet when removing a conversation
    
    When a conversation both has a message removed and eventually evaporates
    or is dropped, it will be attempted to be removed from the conversation
    set twice. This fixes the issue.

 src/engine/app/conversation-monitor/app-conversation-set.vala | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/src/engine/app/conversation-monitor/app-conversation-set.vala 
b/src/engine/app/conversation-monitor/app-conversation-set.vala
index ed44dc72..ca626f6f 100644
--- a/src/engine/app/conversation-monitor/app-conversation-set.vala
+++ b/src/engine/app/conversation-monitor/app-conversation-set.vala
@@ -207,6 +207,7 @@ private class Geary.App.ConversationSet : BaseObject {
                         conversation.to_string()
                     );
                     removed.add(conversation);
+                    remaining.remove(conversation);
                     trimmed.remove_all(conversation);
                     remove_conversation(conversation);
                 } else {