libdwarf: remove use of hide_files()

- This was a nasty workaround due to the way our compiler wrappers used
  to work.  We don't want to have to modify our elfutils installation to
  install libdwarf.

- Since cd9691de5, we no longer need this because the package will always
  come before dependencies in our include order.

lib/spack/llnl/util/filesystem.py

@@ -60,7 +60,6 @@
     'fix_darwin_install_name',
     'force_remove',
     'force_symlink',
-    'hide_files',
     'install',
     'install_tree',
     'is_exe',
@@ -392,18 +391,6 @@ def replace_directory_transaction(directory_name, tmp_root=None):
         tty.debug('TEMPORARY DIRECTORY DELETED [{0}]'.format(tmp_dir))
 
 
-@contextmanager
-def hide_files(*file_list):
-    try:
-        baks = ['%s.bak' % f for f in file_list]
-        for f, bak in zip(file_list, baks):
-            shutil.move(f, bak)
-        yield
-    finally:
-        for f, bak in zip(file_list, baks):
-            shutil.move(bak, f)
-
-
 def hash_directory(directory):
     """Hashes recursively the content of a directory.