Fix :close on terminal panic

Executing :close on a terminal would panic due to it already having been
removed.

This is also related to the fact that removing a tab doesn't check for
whether it actually found a tab to remove or not.

commands/terminal/close.go

@@ -26,6 +26,5 @@ func (_ Close) Execute(aerc *widgets.Aerc, args []string) error {
 	}
 	term, _ := aerc.SelectedTab().(*widgets.Terminal)
 	term.Close(nil)
-	aerc.RemoveTab(term)
 	return nil
 }