CRM-13157 added static to the function and tested isDeleteRelatedMembership use case

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* CRM-13157: isDeleteRelatedMembership() should be declared as static
  http://issues.civicrm.org/jira/browse/CRM-13157
Merge pull request #1347 from davecivicrm/CRM-13157

CRM-13157 added static to the function and tested isDeleteRelatedMembers...