AllowMultipleAddressPrefixesOnSubnet not available

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Multiple address prefixes feature not allowed as of now, so it should be removed, terraform reference to this should also be edited. It is not even in preview. AllowMultipleAddressPrefixesOnSubnet feature needs to be registered for this, which isn't allowed as of now.
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#### Document Details

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* ID: 385e420c-4bd0-3ddd-d9cd-a9a411ec1f7b
* Version Independent ID: 65172372-f485-3d2b-64aa-57c38ac11538
* Content: [az network vnet subnet](https://docs.microsoft.com/en-us/cli/azure/network/vnet/subnet?view=azure-cli-latest#az_network_vnet_subnet_create)
* Content Source: [latest/docs-ref-autogen/network/vnet/subnet.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/master/latest/docs-ref-autogen/network/vnet/subnet.yml)
* Service: **virtual-network**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**