Solr backend: correct usage of pysolr delete

We use HAYSTACK_ID_FIELD in other places but the value passed to
pysolr's delete() method must use the keyword argument ``id``:

https://github.com/toastdriven/pysolr/blob/v3.1.0/pysolr.py#L756

Although the value is passed to Solr an XML tag named ``<id>`` it will
always be checked against the actual ``uniqueKey`` field even if it uses
a custom name:

https://wiki.apache.org/solr/UpdateXmlMessages#A.22delete.22_documents_by_ID_and_by_Query

Closes #943