mcs: deleted scheduler is not synced to scheduling server

[lhy1024]

Enhancement Task

I try add some tests, but failed

pd/tests/integrations/mcs/scheduling/server_test.go

Lines 267 to 277 in 6719a08

	// Fixme: the following code will fail because the scheduler is not removed but not synced.
	// checkDelete := func(schedulerName string) {
	// re.NotNil(schedulersController.GetScheduler(schedulers.BalanceLeaderName) != nil)
	// api.MustDeleteScheduler(re, suite.backendEndpoints, schedulers.BalanceLeaderName)
	// testutil.Eventually(re, func() bool {
	// return schedulersController.GetScheduler(schedulers.BalanceLeaderName) == nil
	// })
	// }
	// checkDelete(schedulers.BalanceLeaderName)
	// checkDelete(schedulers.BalanceRegionName)
	// checkDelete(schedulers.HotRegionName)