Do no use tenant key on S3 KMS SSE when uploading deletion marker

[alanprot]

What this PR does:

This marker is used by cortex to clean up the tenant files after deletion and so should still be accessible by cortex after the deletion.

This file also only store the time of the deletion (non sensitive), and so, i don't see any reason why we need this to be encrypted tenant key.

type TenantDeletionMark struct {
	// Unix timestamp when deletion marker was created.
	DeletionTime int64 `json:"deletion_time"`

	// Unix timestamp when cleanup was finished.
	FinishedTime int64 `json:"finished_time,omitempty"`
}

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• [NA] Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]