backend/s3: Remove deprecated assume role attributes

[gdavison]

Removes deprecated fields from S3 backend for assuming IAM roles outside of the assume_role block.

Target Release

1.10.0

Draft CHANGELOG entry

UPGRADE NOTES

• backend/s3: Removes deprecated attributes for assuming IAM role. Must use the assume_role block.

[jar-b]

LGTM 🎉

Needs a changelog entry (or alternatively a note in the upgrade guide?)

% TF_ACC=1 go test ./...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 146.910s

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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