Don't pass tenant_id in location_config requests from storage controller (#7476)

Tested this locally via a simple patch, the `tenant_id` is now gone from
the json.

Follow-up of #7055, prerequisite for #7469.

libs/pageserver_api/src/models.rs

@@ -429,6 +429,7 @@ pub struct StatusResponse {
 #[derive(Serialize, Deserialize, Debug)]
 #[serde(deny_unknown_fields)]
 pub struct TenantLocationConfigRequest {
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub tenant_id: Option<TenantShardId>,
     #[serde(flatten)]
     pub config: LocationConfig, // as we have a flattened field, we should reject all unknown fields in it