Remove async iterator fix when Node 12 goes EOL

[jimmywarting]

Just so we don't forget... Should maybe remove the fix/hack about async iterator that got resolved in node v12+ or something like that once we drop support for node < 14

Maybe async iterator won't even be a issue when we decide to use whatwg streams? 🤔

Originally posted by @jimmywarting in #1222 (comment)

[tekwiz]

Good call. The applicable blocks of code are:

1. Starting with the c8 ignore directive: 8eeeec1#diff-bfe9874d239014961b1ae4e89875a6155667db834a410aaaa2ebe3cf89820556R102
2. Tests: 13cdcf3#diff-5a684c173d69bcc8d9e0e4b50ac83d8eaf640dbd8e141c14d9e1a953c87afbd3R638
3. README documentation: 9cf8c18#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5R305