[AO] Unskip the custom threshold rule executor unit test (#167120)

Fixes #162299
Fixes #162540

## Summary

This PR unskips the custom threshold rule executor unit test and removes
the warning implementation from the BE.

## 🧪 How to test
- Create a custom threshold rule, it should work as before. (Warning
implementation logic was already removed from FE; this PR only removes
the BE implementation.)

x-pack/plugins/infra/server/lib/alerting/metric_threshold/metric_threshold_executor.test.ts

@@ -131,7 +131,6 @@ const setEvaluationResults = (response: Array<Record<string, Evaluation>>) => {
   jest.requireMock('./lib/evaluate_rule').evaluateRule.mockImplementation(() => response);
 };
 
-// FAILING: https://github.com/elastic/kibana/issues/155534
 describe('The metric threshold alert type', () => {
   describe('querying the entire infrastructure', () => {
     afterAll(() => clearInstances());