Curves: Remove option to disable selection

Remove the redundant option to disable selection in order to simplify
the tools and UI, both conceptually and internally.

It was possible to disable curves selection completely by clicking on
the active selection domain. However, that was redundant compared to
just selecting everything by pressing "A". The remaining potential use
could have been saving a selection for later, but that can be done with
more complete attribute editing tools in the future.

source/blender/blenloader/intern/versioning_300.cc

@@ -3846,5 +3846,9 @@ void blo_do_versions_300(FileData *fd, Library * /*lib*/, Main *bmain)
    */
   {
     /* Keep this block, even when empty. */
+    const int CV_SCULPT_SELECTION_ENABLED = (1 << 1);
+    LISTBASE_FOREACH (Curves *, curves_id, &bmain->hair_curves) {
+      curves_id->flag &= ~CV_SCULPT_SELECTION_ENABLED;
+    }
   }
 }