clicking 'refresh fields' removes the selected field from the combobox

- Go to Settings - Configure index pattern
- For the logstash-\* pattern, it fills in `@timestamp` by default.
- click refresh fields

-> this removes the listed timestamp-field and puts the box in a red outline. The user now needs to expand the combo-box to select a time field. This is confusing to the user, as if they did something wrong by clicking the 'refresh fields' button.
