Don't add variables to scope twice

Since the Variable.parse method already adds to current scope,
p_variable_decl doesn't need to do that again.

lesscpy/lessc/parser.py

@@ -380,7 +380,6 @@ class LessParser(object):
         """
         p[0] = Variable(list(p)[1:-1], p.lineno(4))
         p[0].parse(self.scope)
-        self.scope.add_variable(p[0])
 
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