class_docstr: Remove useless statement like:
      - assignment to unused variables
      - remove alone pure statement
      - remove empty if
  
  >>> import gast as ast
  >>> from pythran import passmanager, backend
  >>> pm = passmanager.PassManager("test")
  >>> node = ast.parse("def foo(): a = [2, 3]; return 1")
  >>> _, node = pm.apply(DeadCodeElimination, node)
  >>> print(pm.dump(backend.Python, node))
  def foo():
      pass
      return 1
  >>> node = ast.parse("def foo(): 'a simple string'; return 1")
  >>> _, node = pm.apply(DeadCodeElimination, node)
  >>> print(pm.dump(backend.Python, node))
  def foo():
      pass
      return 1
  >>> node = ast.parse('''
  ... def bar(a):
  ...     return a
  ... def foo(a):
  ...    bar(a)
  ...    return 1''')
  >>> _, node = pm.apply(DeadCodeElimination, node)
  >>> print(pm.dump(backend.Python, node))
  def bar(a):
      return a
  def foo(a):
      pass
      return 1
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code: def used_target(self, node):
      if isinstance(node, ast.Name):
          if node.id in self.blacklist:
              return True
          chain = self.def_use_chains.chains[node]
          return bool(chain.users())
      return True
  
file_path: catboost/contrib/python/pythran/pythran/optimizations/dead_code_elimination.py
imports: from pythran.analyses import PureExpressions, DefUseChains, Ancestors
  from pythran.openmp import OMPDirective
  from pythran.passmanager import Transformation
  import pythran.metadata
  import gast
label_desc: Use image node_id 2 for calling the DeadCodeElimination obj's underlying member method code with example usage: obj.used_target(node) and returns: True, bool, True