Remove unnecessary disk service and delete step

[aeweston-msft]

Changes introduced by this PR

• Remove the disk service from AzureStackHCIVirtualMachineService struct and the delete step from AzureStackHCIVirtualMachineService Delete flow
• Remove the entire disk service which wraps moc-sdk-for-go, as it is unused after the change

I have been working on some task which required a review of the CAPH interactions with MOC and I realized that there is no need for the OS disk delete step.

This is because the OS disk is a resource whose lifecycle is completely managed within the VM service of MOC and is not represented as a VHD (It has a separate resource called VM image which is only referenced internally to MOC AFAIK). So this is essentially a no-op because this operation should always result in not found. I thought it would be good to remove this code and the corresponding disk service entirely to avoid having dead code and unnecessary calls to MOC. I was just wondering before making a PR what the testing expectations are for the CAPH repo.

Test plan

TBD, unsure of the expectations