CRM-13927 - Counties: fix county notice and remove US county SQL file

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* CRM-13927: County notice gone missing
  https://issues.civicrm.org/jira/browse/CRM-13927
CRM-13927 - Counties: don't need to find path to county file

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* CRM-13927: County notice gone missing
  https://issues.civicrm.org/jira/browse/CRM-13927
CRM-13927 - Counties: link to local extensions directory in message

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* CRM-13927: County notice gone missing
  https://issues.civicrm.org/jira/browse/CRM-13927
Merge pull request #3057 from agh1/counties-as-extensions

CRM-13927 fix county notice and point it to extensions