[CDN] Remove built-in command module

[Ptnan7]

Summary

• Remove the built-in az cdn and az afd command module from Azure CLI core after migration to the cdn extension.
• Remove core command index/service metadata mappings and the azure-mgmt-cdn dependency/resource type mapping.
• Replace Synapse name availability request usage with the Synapse SDK model so core no longer depends on the CDN SDK.

Testing

• azdev linter synapse --rule-types command_groups commands help_entries params
• az cdn -h
• az afd -h
• az cdn profile list -h
• az afd profile list -h

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[yonzhan]

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