Remove unnecessary s:prefix from

Remove unnecessary s:prefix from

From the `synIDattr` help:

    {mode} can be "gui", "cterm" or "term", to get the attributes
    for that mode. When {mode} is omitted, or an invalid value is
    used, the attributes for the currently active highlighting are
    used (GUI, cterm or term).

So, we don't need the explicit s:prefix here; behavior should be
unchanged.