fix(organizations): remove dead metric query param from billing usage

Schema stopped supporting this on the org-level endpoint (FINUB-1907,
2026-07-09). Server never read it; sending it was silently a no-op.

Ref: APIX-1679

Author: vaishakdinesh

organizations/billingusage.go

@@ -317,8 +317,6 @@ type BillingUsageGetParams struct {
 	// period (when consumption happened), not billing period. The maximum date range
 	// is 31 days.
 	From param.Field[time.Time] `query:"from" format:"date"`
-	// Filter results by billable metric id (e.g., workers_standard_requests).
-	Metric param.Field[string] `query:"metric"`
 	// End date for the usage query (ISO 8601). Required if `from` is set. When omitted
 	// along with `from`, defaults to today. Filters by charge period (when consumption
 	// happened), not billing period. The maximum date range is 31 days.