Remove unnecessary document. (#5240)

docs/en/guides/README.md

@@ -114,8 +114,6 @@ haven't provide. You need to
 add a new receiver rather than choosing [existed receiver](../setup/backend/backend-receivers.md).
 At that moment, 
 you most likely need to add a new source and scope. This document will teach you how to do.
-- [Backend Inventory entity extension](inventory-extension.md). If you want to extend SkyWalking inventory entities, and
-want to push upstream back to our Apache OSS repo, please read these principles.
 
 ### UI developer
 Our UI is constituted by static pages and web container.

docs/en/guides/inventory-extension.md

@@ -1,31 +0,0 @@
-# Backend Inventory Entity Extension
-SkyWalking includes four inventory entities.
-- Service Inventory
-- Service Instance Inventory
-- Endpoint Inventory
-- Network Address Inventory
-
-All metrics, topology, trace and alarm are related to these entity IDs. 
-
-For understanding the **Service**, **Service Instance** and **Endpoint** concepts,
-please read [Project Overview](../concepts-and-designs/overview.md#why-use-skywalking).
-
-For **Network Address Inventory**, it represents all network address, in IP:port, hostname, domain name
-formats, which are detected by language agents or other probes.
-
-## Extension
-Right now, only **Service Inventory** extension is already supported in backend core.
-Service provides field `properties` in Json format, which is usually used for specific service 
-rather than normal business services, such as Database, Cache, MQ, etc.
-
-For keeping code consistent and friendly in query and visualization, the Json properties
-need to follow the rules.
-
-### Database
-1. NodeType == **Database(1)**
-1. Json properties include following keys.
-  - `database`. Database name, such as MySQL, PostgreSQL
-  - `db.type`. Database type, such as sql db, redis db.
-  - `db.instance`. Database instance name.
-
-