Variants: remove the presentation of the default variant

The concept of a "default variant" was not particular the intention,
but rather the side effect of the implementation.  To make things
clearer, let's not present any variant if none was actively created by
any plugin implementation.

Further work will change the internals to completely remove its
existence.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/varianter.py

@@ -265,15 +265,7 @@ class Varianter:
         :rtype: str
         """
         if self._no_variants == 0:  # No variants, only defaults:
-            out = []
-            if summary:
-                out.append("No variants available, using defaults only")
-            if variants:
-                variant = next(self.itertests())
-                variant["variant_id"] = ""  # Don't confuse people with None
-                out.append("\n".join(variant_to_str(variant, variants - 1,
-                                                    kwargs, self.debug)))
-            return "\n\n".join(out)
+            return ""
 
         out = [item for item in self._variant_plugins.map_method_with_return("to_str",
                                                                              summary,