scheduler: remove the restriction that reservation cannot be preempted #7253

Job	Run time
golangci-lint	1m 34s
unit-tests(Check Go modules)	30s
unit-tests(Check manifests)	37s
unit-tests(Check auto-generated codes)	4m 32s
unit-tests(Verify gofmt)	38s
unit-tests(Verify govet)	1m 26s
unit-tests(Run Go build)	1m 16s
unit-tests(Run Go test)	3m 44s
build-and-push (koordlet)	3m 41s
build-and-push (koord-manager)	2m 14s
build-and-push (koord-scheduler)	3m 17s
build-and-push (koord-descheduler)	2m 41s
	26m 10s

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