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

tools/gn/string_utils.cc

Index: tools/gn/string_utils.cc
diff --git a/tools/gn/string_utils.cc b/tools/gn/string_utils.cc
index 6d3f87774a04be5d3ff4b6aa46f651e84da92409..496970bcc00819c9c4da77f381d28259d967107c 100644
--- a/tools/gn/string_utils.cc
+++ b/tools/gn/string_utils.cc
@@ -242,16 +242,3 @@ bool ExpandStringLiteral(Scope* scope,
   }
   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);
-  }
-}