[CLI/PS] KeyVault -Remove client-side RBAC assignment creation logic for Key Vault and rely on service API behavior

**Background**

Currently, Azure CLI and Azure PowerShell implement custom client-side logic to handle Key Vault RBAC authorization during Key Vault creation. This logic predates the service API support and was introduced to provide a consistent RBAC experience for users.

The Key Vault service API now natively supports RBAC authorization configuration as part of the resource creation workflow. Maintaining client-side logic creates duplicate behavior, increases maintenance cost, and introduces the risk of divergence between client tools and service implementation.

**Problem Statement**

Azure CLI and Azure PowerShell currently override or supplement Key Vault RBAC configuration through client-side implementation.

Remove the client-side RBAC assignment/configuration logic from Azure CLI and Azure PowerShell.

**Instead:**

Pass user-specified parameters directly to the Key Vault service API.
Rely on the service-defined API contract and behavior for RBAC configuration.
Eliminate any client-specific RBAC handling that duplicates service functionality.
Ensure client tools remain aligned with the latest Key Vault API implementation.