Revert "Add ::selection to root color vars"

[keithamus]

Reverts #2437

@langermank This PR will need reverting because the introduction of ::selection blows the whole selector away. The ::selection style needs to be standalone it seems. In particular, the following:

@include color-mode("light") {
  p {
    color: green
  }
}

gets compiled into

:root p,
::selection p {
  color: green
}

which it seems is discarded. Here's a minimal repro to demonstrate that the selector is discarded:

https://codepen.io/keithamus/pen/mdQrGMo

Refs https://github.com/github/issues/issues/6741, https://github.com/orgs/community/discussions/58723#discussioncomment-6250327

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