chore!: remove XaxrMathExpression construct

The upstream MathExpression supports this now.

Author: Eugene Cheung

API.md

@@ -4,4 +4,3 @@ export * from "./MetricFactory";
 export * from "./MetricStatistic";
 export * from "./MetricWithAlarmSupport";
 export * from "./RateComputationMethod";
-export * from "./XaxrMathExpression";