Remove Http2String wrapper for std::string.

gfe-relnote: n/a, no functional change.
PiperOrigin-RevId: 263816460
Change-Id: I4f365540414e6a3d78fd00ce6b39a4cdd4c14140
diff --git a/http2/hpack/decoder/hpack_string_collector.cc b/http2/hpack/decoder/hpack_string_collector.cc
index 247ce9c..e903755 100644
--- a/http2/hpack/decoder/hpack_string_collector.cc
+++ b/http2/hpack/decoder/hpack_string_collector.cc
@@ -36,7 +36,7 @@
   Clear();
 }
 
-HpackStringCollector::HpackStringCollector(const Http2String& str, bool huffman)
+HpackStringCollector::HpackStringCollector(const std::string& str, bool huffman)
     : s(str), len(str.size()), huffman_encoded(huffman), state(kEnded) {}
 
 void HpackStringCollector::Clear() {
@@ -89,7 +89,7 @@
   return ::testing::AssertionSuccess();
 }
 
-Http2String HpackStringCollector::ToString() const {
+std::string HpackStringCollector::ToString() const {
   std::stringstream ss;
   ss << *this;
   return ss.str();