CRM-18472 - CRM_Contact_BAO_GroupContact - Remove erroneous references

CRM-18472 - CRM_Contact_BAO_GroupContact - Remove erroneous references

These lines are attempting to accept a return-value by reference, but the
function does not return by reference. In PHP 5.3+, this is effectively
not a reference; and in PHP 5.4+, it generates a warning. That leads
to test failures in CRM_Contact_BAO_ContactTest::testCreateProfileContact.