docs: public void clear(String cacheKey) {
          ValueCacheKey key = getRequestCacheKey(cacheKey);
          if (key != null) {
              ConcurrentHashMap<ValueCacheKey, HystrixCachedObservable<?>> cacheInstance = requestVariableForCache.get(concurrencyStrategy);
              if (cacheInstance == null) {
                  throw new IllegalStateException("Request caching is not available. Maybe you need to initialize the HystrixRequestContext?");
              }
  
              /* remove this cache key */
              cacheInstance.remove(key);
          }
      }