make unit_type an optional parameter when deleting files

refs #7357

the connect_and_remove_transfer_files method requires two parameters,
the uuid of the transfer or sip that you want to remove from the
ElasticSearch index, and the unit_type, which can be either
'sip' or 'transfer'.

This change makes the unit_type parameter optional (default is None)
that way it is not necessary to know if the package you want to remove
is a sip or a transfer.  The uuid uniquely identifies it, so
the unit_type does not help guarantee a match in any case.
use generic version of elasticSearch remove files method

refs #7357

When deleting files from the ElasticSearch index, take
advantage of the change in commit f31955ac58fb5a1fba3ef3c432ee752956130beb
that provides a more generic remove method.