Remove PremiumRS edition value due to public preview schedule changes

[hshihMS]

Description

Due to changes on the schedule of PremiumRS edition, the management has decided to delay its exposure through PowerShell. Thus the enum is removed for now and will be added later when the edition is released.

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[azurecla]

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[markcowl]

on demand run here:http://azuresdkci.cloudapp.net/view/1-AzurePowerShell/job/powershell-demand/1252/

LGTM once on-demand run passes