Kindsys: Replace DefForGen with kindsys.Kind (#62642)

* Kindsys: Replace DeclForGen with kindsys.Kind

DeclForGen was always unnecessary - it just wasn't obvious on initial
implementation, when we were focused on generating unique types for each
core kind. This removes it, considerably simplifying interactions with
kindsys - virtually everything now just relies on kindsys.Kind and its
derived interfaces.

* Removed unused jenny

* Rename params in jennies

pkg/codegen/tmpl.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/grafana/codejen"
+	"github.com/grafana/grafana/pkg/kindsys"
 )
 
 // All the parsed templates in the tmpl subdirectory
@@ -40,7 +41,7 @@ type (
 		// Header tvars_autogen_header
 		PackageName       string
 		KindPackagePrefix string
-		Kinds             []*DefForGen
+		Kinds             []kindsys.Kind
 	}
 	tvars_coremodel_imports struct {
 		PackageName string