Clear non-language specific highlights fields in Solr query

Currently, the apachesolr_multilingual alters the Solr query by clearing all highlights fields (hl.fl parameter) and reassigning language specific ones. However, adding/changing hl.fl parameters in other hook implementations, such as hook_apachesolr_query_prepare() or hook_apachesolr_query_alter() are overridden.

This patch only clear the non-language specific hl.fl fields leaving the language specific fields untouched.