Chore: Refactor backend plugin manager/tsdb query data (#34944)

Move QueryData method into backend plugin manager which HandleRequest uses to 
query data from plugin SDK supported data sources. This allowed us to remove a lot 
of code no longer needed.

Ref #21510

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

pkg/expr/service_test.go

@@ -113,7 +113,3 @@ func (me *mockEndpoint) DataQuery(ctx context.Context, ds *models.DataSource, qu
 type fakeBackendPM struct {
 	backendplugin.Manager
 }
-
-func (pm fakeBackendPM) GetDataPlugin(string) interface{} {
-	return nil
-}