Fix TestAccEventarcGoogleChannelConfig_cryptoKeyUpdate to properly clean up

[modular-magician]

fixes hashicorp/terraform-provider-google#12908

After making updates to the way we manage permissions during tests, we still saw issues with a few TestAccCloudfunctions2function* tests. It turns out TestAccEventarcGoogleChannelConfig_cryptoKeyUpdate was leaving a region-wide configuration on the project that enforced CMEK on all eventarc channels, which were being used in these cloudfunction tests. See https://cloud.google.com/eventarc/docs/use-cmek.

To fix our tests, I've updated TestAccEventarcGoogleChannelConfig_cryptoKeyUpdate to clean up its region-wide CMEK when it is finished. Note that this apparently does not happen automatically on destroy, which is why I needed an additional step to clear the config with an update.

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Derived from GoogleCloudPlatform/magic-modules#7479