DELETE Microsoft.Authorization/roleAssignments may also return HTTP code 204

**Issue:** According to [Microsoft-internal telemetry](https://microsoft-my.sharepoint-df.com/:t:/p/pevezzac/ESMDfu51bOpKtMGui7r-2DUBO5lQ7pb6YyZSLW3kQXvNDA?e=dTgjqL), a `DELETE` operation for the `Microsoft.Authorization/roleAssignments` endpoint may also return HTTP code 204. This may returned when the same `DELETE` operation is triggered in a quick succession.

Currently all API versions for the `Microsoft.Authorization` provider list only HTTP code 200 as a valid success return code:

* [2015-07-01](https://github.com/Azure/azure-rest-api-specs/blob/bd9c2010c4165f1e792d6fec1e40c5fde49d063e/specification/authorization/resource-manager/Microsoft.Authorization/stable/2015-07-01/authorization-RoleAssignmentsCalls.json#L190-L197)
* ...
* [2020-04-01-preview](https://github.com/Azure/azure-rest-api-specs/blob/bd9c2010c4165f1e792d6fec1e40c5fde49d063e/specification/authorization/resource-manager/Microsoft.Authorization/preview/2020-04-01-preview/authorization-RoleAssignmentsCalls.json#L178-L185)

This causes downstream problems as AutoRest clients depend on the API spec to parse the return code and will throw an exception when facing unexpected HTTP codes such as 204 e.g. https://github.com/Azure/azure-libraries-for-net/issues/1139

**Expected:** Either add a new REST API spec for the `Microsoft.Authorization` provider or edit the existing ones including HTTP 204 as a valid return code.

Start with specification/authorization/resource-manager/Microsoft.Authorization/stable/2015-07-01/authorization-RoleAssignmentsCalls.json and the preview/2020-04-01-preview equivalent linked in the issue, then inspect the other Microsoft.Authorization API versions. Confirm how DELETE roleAssignments responses are declared and update the relevant specifications so HTTP 204 is accepted alongside 200; done means the affected versions consistently describe both success codes.

Written by the indexing model from the issue text.