rBa8f7d41d3898 added a "duplicate" check for being in curves sculptmode
unnecessarily afaict (in_sculpt_curve_mode in addition to the
previously existing in_curves_sculpt_mode).
Over time, the later evolved to also take into account the output of a
viewer node, see rBc55d38f00b8c (the previously existing
in_curves_sculpt_mode did not receive this).

This all results in the fact that selection is not drawn with a viewer
node (which might even be intended? -- can be useful though, and there
are separate opacity controls for both selection and the viewer
attribute, so these can be used/blended to everyones liking).

So now deduplicate the check.