chore(oidc): remove legacy storage methods (#10061)

# Which Problems Are Solved

Stabilize the optimized introspection code and cleanup unused code.

# How the Problems Are Solved

- `oidc_legacy_introspection` feature flag is removed and reserved.
- `OPStorage` which are no longer needed have their bodies removed.
- The method definitions need to remain in place so the interface
remains implemented.
  - A panic is thrown in case any such method is still called

# Additional Changes

- A number of `OPStorage` methods related to token creation were already
unused. These are also cleaned up.

# Additional Context

- Closes #10027 
- #7822

---------

Co-authored-by: Livio Spring <livio.a@gmail.com>

internal/api/oidc/userinfo.go

@@ -35,9 +35,6 @@ func (s *Server) UserInfo(ctx context.Context, r *op.Request[oidc.UserInfoReques
 	}()
 
 	features := authz.GetFeatures(ctx)
-	if features.LegacyIntrospection {
-		return s.LegacyServer.UserInfo(ctx, r)
-	}
 	if features.TriggerIntrospectionProjections {
 		query.TriggerOIDCUserInfoProjections(ctx)
 	}