Remove extra validation of the sku names on create new capacity

[selasagi]

Description

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[maddieclayton]

@selasagi Is this change happening because the ValidateSet represents values that will always be in the list of available skus? I just want to make sure I fully understand this change.

[selasagi]

Yes, the ValidateSet represents values that will always be in the list of available skus but we have now bug which that the available skus API returns empty list for some subscriptions. I mean that for now part of our users can't use that cmdlet for every sku value.
That is the reason this fix is so important!

[selasagi]

We fix the bug in the available skus API but it is not in production yet (and it will take more time because of the holidays..)

[maddieclayton]

on demand here: https://azuresdkci.westus2.cloudapp.azure.com/view/PowerShell/job/powershell-demand/305/

[maddieclayton]

@azuresdkci Test this please