apachesolr_multisitesearch_apachesolr_delete_by_query_alter definition not passing query by reference

apachesolr_multisitesearch_apachesolr_delete_by_query_alter() should be defined as passing the $query variable by reference. Given that the $query variable is a string and not an object, we must declare that it should be passed by reference. The apachesolr documentation is incorrect, but an issue (#1996366: documenation for hook_apachesolr_delete_by_query_alter should define $query as being passed by reference) has been raised to fix that.

The attached patch fixes the issue for apachesolr_multisitesearch.