backend/sqlstore: move GetDashboardTags to dashboard service (#50091)

+ removing some functions that were already copied to the dashboard service in a previous pr.

pkg/services/dashboards/database/database_test.go

@@ -292,7 +292,7 @@ func TestIntegrationDashboardDataAccess(t *testing.T) {
 		setup()
 		query := models.GetDashboardTagsQuery{OrgId: 1}
 
-		err := sqlStore.GetDashboardTags(context.Background(), &query)
+		err := dashboardStore.GetDashboardTags(context.Background(), &query)
 		require.NoError(t, err)
 
 		require.Equal(t, len(query.Result), 2)