Chore: Remove unused Go code (#28852)

* Chore: Remove more unused Go code

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/plugins/app_plugin.go

@@ -13,11 +13,6 @@ import (
 	"github.com/grafana/grafana/pkg/util/errutil"
 )
 
-type AppPluginCss struct {
-	Light string `json:"light"`
-	Dark  string `json:"dark"`
-}
-
 type AppPlugin struct {
 	FrontendPluginBase
 	Routes []*AppPluginRoute `json:"routes"`