doc: remove stability highlight for stable functions

We don't have the stability functions for all stable
functions, only those that were previously experimental and
then when stable. Remove the stability indicator to be
consistent with the rest of the functions.

PR-URL: https://github.com/nodejs/node/pull/27753


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>