[Identity] Remove AzureApplicationCredential

This type of credential was added back in[ 1.5.0-beta.3 (2021-08-10)](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/identity/Azure.Identity/CHANGELOG.md#features-added-18) and then hidden on October 2021 https://github.com/Azure/azure-sdk-for-net/pull/24647.

We should remove it completely from the repo since it has not been exposed since that beta.