Stop deleting indices on module uninstall

Elasticsearch helper implements hook_module_uninstall which deletes the respective indices when uninstalling a module providing an elasticsearch helper index plugin.

Imagine two modules providing index plugins which point to the same index but for different types. Uninstalling one of them would accidently delete the index controlled by the other module.

I guess the only save way to handle this is to not automatically delete an index when uninstalling a module.

Patch follows.