fix: MPIJob cannot use gang-scheduling when --enable-gang-scheduling is set

[cheimu]

What this PR does / why we need it:
Thanks for @zw0610 pointing out, the mpijob-operator logic where if gang-scheduler is set is totally opposite. Currently, if gang-scheduler name is not set, then throws an error and directly use default-scheduler instead. However, the correct logic should be if util.IsGangSchedulerSet is true, then throw error and use user's indicated scheduler; else the mpijob-operator should render a default volcano for schedulerName field. So this pr remove ! when check if gang scheduler is set or not

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #1548

Checklist:

• Docs included if any changes are user facing

[aws-kf-ci-bot]

Hi @cheimu. Thanks for your PR.

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[coveralls]

Pull Request Test Coverage Report for Build 2014670313

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 36.921%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	2	75.35%

Totals
Change from base Build 2014597889:	0.02%
Covered Lines:	2278
Relevant Lines:	6170

---

💛 - Coveralls

[zw0610]

/ok-to-test

[zw0610]

/lgtm

[terrytangyuan]

Thanks!

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cheimu, terrytangyuan

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