Add support for delete vector index operation

[antas-marcin]

Add a possibility of removing a vector index the same way as delete property index was implemented.

REST API endpoint definition: DELETE /v1/schema/{className}/vectors/{vectorIndexName}/index

Here's a link to swagger endpoint definition: https://github.com/weaviate/weaviate/blob/alter-schema-drop-vector-index/openapi-specs/schema.json#L8195-L8249