sample: remove AwsCrossAccountRole from AWS sample.

[TAnchor]

fix: remove AwsCrossAccountRole from AWS sample.

This was updated in https://github.com/googleapis/java-bigqueryconnection/pull/576/files, but one reference to AwsCrossAccountRole was not changed, which causes the sample to not work as the cross account role is no longer supported.

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Fixes #<issue_number_goes_here> ☕️

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