Fix remove method in AvlTree: Nodes can not re-balance.

[Bladefidz]

Remove node in AvlTree with auto balancing.

Fix issue: #13

[codecov-io]

Codecov Report

Merging #33 into master will decrease coverage by 0.31%.
The diff coverage is 73.91%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage     100%   99.68%   -0.32%     
==========================================
  Files          86       86              
  Lines        1883     1906      +23     
  Branches      332      338       +6     
==========================================
+ Hits         1883     1900      +17     
- Misses          0        6       +6

Impacted Files	Coverage Δ
src/data-structures/tree/avl-tree/AvlTree.js	92.1% <73.91%> (-7.9%)	⬇️

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[trekhleb]

Wow, @Bladefidz , thank you for your huge effort! Amount of code is impressive!

I've already pushed Red-Black-Tree implementation though. The main difference between the code in master branch and this PR is that in master the Red-Black tree is based upon Binary Search tree. I think it is good approach to prevent code duplication and also because Red-Black trees are BSTs by definition.

Could you please re-target this pull request so that it purpose would be balancing AVL tree while removing the node from it?

[Bladefidz]

Okay @trekhleb, I will modify my PR so only commits on AVLTree will be pulled.

[trekhleb]

@Bladefidz thank you for PR!