Plugins: Remove old code related to Core plugin installs (#44311)

* remove old code

* remove even more

* skip flaky test

pkg/plugins/manager/store.go

@@ -2,12 +2,10 @@ package manager
 
 import (
 	"context"
-	"fmt"
 	"path/filepath"
 	"strings"
 
 	"github.com/grafana/grafana/pkg/plugins"
-	"github.com/grafana/grafana/pkg/plugins/backendplugin"
 )
 
 func (m *PluginManager) Plugin(_ context.Context, pluginID string) (plugins.PluginDTO, bool) {
@@ -83,28 +81,6 @@ func (m *PluginManager) Add(ctx context.Context, pluginID, version string) error
 	return nil
 }
 
-func (m *PluginManager) AddWithFactory(ctx context.Context, pluginID string, factory backendplugin.PluginFactoryFunc,
-	pathResolver plugins.PluginPathResolver) error {
-	if m.isRegistered(pluginID) {
-		return fmt.Errorf("plugin %s is already registered", pluginID)
-	}
-
-	path, err := pathResolver()
-	if err != nil {
-		return err
-	}
-
-	p, err := m.pluginLoader.LoadWithFactory(ctx, plugins.Core, path, factory)
-	if err != nil {
-		return err
-	}
-	err = m.register(p)
-	if err != nil {
-		return err
-	}
-	return nil
-}
-
 func (m *PluginManager) Remove(ctx context.Context, pluginID string) error {
 	plugin, exists := m.plugin(pluginID)
 	if !exists {