Update LFUCache.js

[shpetimaliu]

• I think It is not necessary to use Object.seal in CacheNode and LFUCache since you are not modifying anything in them after they are created.

• In insert and refresh in FrequencyMap, you don't need to create a new Set if a specific frequency doesn't exist in the Map. It could be created immediately and would be more efficient.

• In LFUCache, instead of calling the methods cache.has(key) and cache.get(key), you can access the cache value directly using this.cache.get(key) and this.cache.has(key). This is shorter and clearer.

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Describe your change:

• [ ] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [ ] Documentation change?

Checklist:

• [ ] I have read CONTRIBUTING.md.
• [ ] This pull request is all my own work -- I have not plagiarized.
• [ ] I know that pull requests will not be merged if they fail the automated tests.
• [ ] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ ] All new JavaScript files are placed inside an existing directory.
• [ ] All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames. Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not
• [ ] All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• [ ] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[appgurueu]

CI is failing.

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