[relay] Add python client name customization and remove obsolete ArmResource rename (#44440)

- Set the Python SDK client name to RelayAPIMgmtClient via @@clientName python override.
- Remove the no-longer-needed @@clientName ArmResource python override from back-compatible.tsp.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: msyyc

specification/relay/resource-manager/Microsoft.Relay/Relay/back-compatible.tsp

@@ -118,9 +118,3 @@ using Microsoft.Relay;
 
 // NamespacesOperationGroup operations
 @@clientLocation(NamespacesOperationGroup.checkNameAvailability, "Namespaces");
-
-@@clientName(
-  Azure.ResourceManager.CommonTypes.Resource,
-  "ArmResource",
-  "python"
-);

specification/relay/resource-manager/Microsoft.Relay/Relay/client.tsp

@@ -4,6 +4,8 @@ import "@azure-tools/typespec-client-generator-core";
 using Azure.ClientGenerator.Core;
 using Microsoft.Relay;
 
+@@clientName(Microsoft.Relay, "RelayAPIMgmtClient", "python");
+
 // Preserve existing "WcfRelays" client name (was "WCFRelays" in TypeSpec)
 @@clientName(WCFRelays, "WcfRelays", "java");