tools/gn: remove three unused functions from string_utils.h

tools/gn/string_utils.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/string_utils.h"
 
 #include "tools/gn/err.h"
 #include "tools/gn/input_file.h"
 #include "tools/gn/parser.h"
 #include "tools/gn/scope.h"
@@ skipping 224 matching lines @@
     } else if (input[i] == '$') {
       if (!AppendStringInterpolation(scope, literal, input, size, &i,
                                      &output, err))
         return false;
     } else {
       output.push_back(input[i]);
     }
   }
   return true;
 }
-
-std::string RemovePrefix(const std::string& str, const std::string& prefix) {
-  CHECK(str.size() >= prefix.size() &&
-        str.compare(0, prefix.size(), prefix) == 0);
-  return str.substr(prefix.size());
-}
-
-void TrimTrailingSlash(std::string* str) {
-  if (!str->empty()) {
-    DCHECK((*str)[str->size() - 1] == '/');
-    str->resize(str->size() - 1);
-  }
-}