Delete Solr index

1. GET https://opensolr.com/solr_manager/api/delete_core
2. Parameters:
   1. ​email - your opensolr registration email address
   2. api_key - your opensolr api_key
   3. ​index_name - the name of the core you wish to delete

3. Example: https://opensolr.com/solr_manager/api/delete_index?email=PLEASE_LOG_IN&
   api_key=PLEASE_LOG_IN&index_name=my_solr_index