[clang-tidy] Remove redundant const

Found with readability-const-return-type

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/Lexer.cpp

@@ -114,7 +114,7 @@ std::vector <std::string> Lexer::split (const std::string& text)
 
 ////////////////////////////////////////////////////////////////////////////////
 // No L10N - these are for internal purposes.
-const std::string Lexer::typeName (const Lexer::Type& type)
+std::string Lexer::typeName (const Lexer::Type& type)
 {
   switch (type)
   {