Revert "avocado.core.multiplexer: Assign tag to match multiplex variant"

This reverts commit 913ab2ab.

Turns out that change appends inconditionally the tags to tests,
even the ones not using the multiplexing system. We need to revert
and figure a better solution.

avocado/core/multiplexer.py

@@ -421,7 +421,6 @@ class Mux(object):
             i = None
             for i, variant in enumerate(self.variants):
                 test_factory = [template[0], template[1].copy()]
-                test_factory[1]['tag'] = "variant%s" % (i + 1)
                 inject_params = test_factory[1].get('params', {}).get(
                     'avocado_inject_params', False)
                 # Test providers might want to keep their original params and