bug: aria-hidden=true should be removed

[titanism]

This line needs to be removed: https://github.com/Flet/markdown-it-github-headings/blob/d4c008d06ee692a6a564858bd78e63bd58e8ace0/index.js#L62

Note that you already have aria-hidden=true on the SVG, which should suffice.

Google's PageSpeed Insights deems this current behavior negatively.

[titanism]

Someone should probably also notify GitHub support of this since this is mirrored from them.