Revert "Allow using literals namespaces"

This reverts commit 73e0f3b.

Author: tkruse

cpplint.py

@@ -4893,7 +4893,7 @@ def CheckLanguage(filename, clean_lines, linenum, file_extension,
           'Did you mean "memset(%s, 0, %s)"?'
           % (match.group(1), match.group(2)))
 
-  if Search(r'\busing namespace\b', line) and not Search(r'\bliterals\b', line):
+  if Search(r'\busing namespace\b', line):
     error(filename, linenum, 'build/namespaces', 5,
           'Do not use namespace using-directives.  '
           'Use using-declarations instead.')