Remove graduated github api preview headers

[jbpratt]

The symmetra and hellcat preview headers are no longer required

https://developer.github.com/changes/2019-12-13-graduated-previews-announced/

https://docs.github.com/en/rest/overview/media-types

resolves #27757

Signed-off-by: jbpratt jbpratt78@gmail.com

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[petr-muller]

/ok-to-test

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