chore: remove the spec rule support in redocly.yaml (#2092)

Author: tatomyr

packages/core/src/types/redocly-yaml.ts

@@ -172,7 +172,6 @@ const builtInRules = [
   ...builtInAsync3Rules,
   ...builtInArazzo1Rules,
   ...builtInOverlay1Rules,
-  'spec', // TODO: depricated in favor of struct
   'struct',
 ] as const;