sysvar: delete two sysvar that has been removed from code (pingcap#11305)

Author: TomShawn

system-variables.md

@@ -704,22 +704,6 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 
 </CustomContent>
 
-### tidb_auth_signing_cert
-
-- Scope: GLOBAL
-- Persists to cluster: Yes
-- Type: String
-- Default value: ""
-- This variable is associated with an unreleased feature. **DO NOT** set this variable.
-
-### tidb_auth_signing_key
-
-- Scope: GLOBAL
-- Persists to cluster: Yes
-- Type: String
-- Default value: ""
-- This variable is associated with an unreleased feature. **DO NOT** set this variable.
-
 ### tidb_auto_analyze_end_time
 
 - Scope: GLOBAL