Mark 'Number of participants' as required; remove unused reqMark

Sorry to bundle two issues together, but the aim of this issue is to:

• Add the required marker to the 'Number of participants' field for event registration. Technically it is a required field, so should be marked as such.

• As a minor side issue, remove the unused 'reqMark' variable from all templates.

Source of this issue was this on stack exchange: http://civicrm.stackexchange.com/questions/2614/how-to-add-a-class-to-a-form-element-in-buildquickform/2617