this.ttl is always `undefined` on `LRUCache.js`

[yomybaby]

Hi, I found some thing which is never used.

Please check the 16th line of LRUCache.js.

this.ttl is always undefined. it is never assigned. Even this is undefined, LRU use default TTL when put.(https://www.npmjs.com/package/lru-cache#api) So undefined doesn't make problems.

But this.ttl is never used. So it should be removed.

[stale]

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