fix: un-assign agent from conversation after removal (#5417)

Unassign agents from the conversation when they are removed from the account.

Fixes: #4555

spec/services/agents/destroy_service_spec.rb

@@ -9,15 +9,17 @@ describe Agents::DestroyService do
   before do
     create(:team_member, team: team1, user: user)
     create(:inbox_member, inbox: inbox, user: user)
+    create(:conversation, account: account, assignee: user, inbox: inbox)
   end
 
   describe '#perform' do
-    it 'remove inboxes and teams when removed from account' do
+    it 'remove inboxes, teams, and conversations when removed from account' do
       described_class.new(account: account, user: user).perform
       user.reload
       expect(user.teams.length).to eq 0
       expect(user.inboxes.length).to eq 0
       expect(user.notification_settings.length).to eq 0
+      expect(user.assigned_conversations.where(account: account).length).to eq 0
     end
   end
 end