CRM-20420 - PDFLetterCommon::createActivities() - Simplify. Address comment.

CRM-20420 - PDFLetterCommon::createActivities() - Simplify. Address comment.

The comment asks why the code appears to use of `$contactIds` and
`$form->contactIds` interchangeably.  If you grep for calls to
`createActivities()`, all 3-ish callpaths do set `$contactIds` based on
`$form->contactIds` -- so they are the same.

```
// Look for calls to createActivities()
$ grep -r ::createActivities CRM/ Civi/ api/
CRM/Contact/Form/Task/PDFLetterCommon.php:      self::createActivities($form, $html_message, $form->_contactIds);
CRM/Member/Form/Task/PDFLetterCommon.php:    self::createActivities($form, $html_message, $contactIDs);
CRM/Contribute/Form/Task/PDFLetterCommon.php:    self::createActivities($form, $html_message, $form->_contactIds);

// Look for calls to createActivities via CRM/Member/Form/Task/PDFLetterCommon

$ rgrep -r postProcessMembers\( CRM/ Civi/ api/
CRM/Member/Form/Task/PDFLetter.php:    CRM_Member_Form_Task_PDFLetterCommon::postProcessMembers(
CRM/Member/Form/Task/PDFLetterCommon.php:  public static function postProcessMembers(&$form, $membershipIDs, $skipOnHold, $skipDeceased, $contactIDs) {
```