DRUP-789 Fix PHP notice caused by missing array key

[Jaesin]

"obscured" array key should not be removed because \Drupal\key\Form\KeyFormBase::submitForm() expects that it exist.

Logic fix from #186.

[googlebot]

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[mxr576]

@cnovak Please post-verify the CLAs, but they should be ok. What is wrong with the Googlebot? :)

[cnovak]

I thought Googlebot would disallow merge. It cannot verify when there is more than one author in a PR. Verified

[mxr576]

Meh, that is an odd limitation of the service ;S