removed and tested unecessary gas cost

[YZhenY]

Fixes #977

🚀 Description

Added tests for behaviour of removeTokenFrom.
Removed redundant line from ERC721 contract to improve gas cost

[come-maiz]

Thanks @YZhenY!
I was doing some tests on the solidity behavior, and confirmed that when you decrease the length of an array and increase it again, the values are reset to zero:
https://github.com/elopio/solidity-experiments/blob/28d622aabea4492032916be9dab0627b15b6118c/test/Arrays.test.js#L62

So, I don't see any reason reset the value before reducing the length, not even clearing traces. I'm +1 here, but can you please revert your changes to package-lock.json?

[nventuro]

I restored the original package-lock.json file and added a clarifying comment, thanks for your contribution @YZhenY!