[2021.2 Backport] Fix error when using gizmos with camera stack in editor

[tomzig16]

Purpose of this PR

Backport for #5913

This PR fixes: https://fogbugz.unity3d.com/f/cases/1394156/
Previously there was an assert that was used as camera stack was not implemented, but now it works with it. When PR was made to change the if statement, assertion was forgotten hence it causes the regression. After removing it everything works correctly.

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Testing status

Tested locally + automated ABV tests

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Comments to reviewers

Although bug report tells that it is not a regression, we found that the issue appeared indirectly after the changes made in #4034

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[martint-unity]

Looks good to me