Comment-out several redundant hl-group links in:

- lua/github-theme/group/syntax.lua
- lua/github-theme/group/modules/treesitter.lua

These are redundant because they specify links which are identical to
Neovim defaults (e.g. `@string --> String`, `@function.call -->
@function`, etc.). They are mimicking default links that Neovim already
ships and starts-up with.

Colorschemes may break Neovim's default links arbitrarily, and `:hi clear`
restores them. Since we don't know if/which default links have been
broken by the previous colorscheme when loading ours, we should restore
these default links—either with `:hi clear`, or manually—before ours is
loaded. This already appears to be happening within compiled
colorschemes (i.e. `hi clear` gets embedded within them). Furthermore,
colorscheme compilation happens automatically by default and cannot be
disabled.

Syntax highlighting is still not 100% accurate (due mostly to
highlighting differences between different languages which require
particular consideration), however, the accuracy of the highlights have
been improved, both overall, and in terms of common languages such as:
rust, ruby, ecma, c, c#, go, html, css, make, python, and lua.

See projekt0n#252