fix: remove alias for ng doc command

Backport #16368 to 8.x branch

Closes: #16825

Author: alan-agius4

packages/angular/cli/commands/doc.json

@@ -4,7 +4,6 @@
   "description": "Opens the official Angular documentation (angular.io) in a browser, and searches for a given keyword.",
   "$longDescription": "",
 
-  "$aliases": [ "d" ],
   "$type": "native",
   "$impl": "./doc-impl#DocCommand",