[xDS] remove thread-unsafe caching of locality names (#35862)

b/324102797

Closes #35862

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35862 from markdroth:lb_endpoint_iterator_crash_fix 74929498a8
PiperOrigin-RevId: 605462070

src/core/ext/xds/xds_client_stats.h

@@ -86,13 +86,9 @@ class XdsLocalityName : public RefCounted<XdsLocalityName> {
   const std::string& zone() const { return zone_; }
   const std::string& sub_zone() const { return sub_zone_; }
 
-  const std::string& AsHumanReadableString() {
-    if (human_readable_string_.empty()) {
-      human_readable_string_ =
-          absl::StrFormat("{region=\"%s\", zone=\"%s\", sub_zone=\"%s\"}",
-                          region_, zone_, sub_zone_);
-    }
-    return human_readable_string_;
+  std::string AsHumanReadableString() const {
+    return absl::StrFormat("{region=\"%s\", zone=\"%s\", sub_zone=\"%s\"}",
+                           region_, zone_, sub_zone_);
   }
 
   // Channel args traits.
@@ -108,7 +104,6 @@ class XdsLocalityName : public RefCounted<XdsLocalityName> {
   std::string region_;
   std::string zone_;
   std::string sub_zone_;
-  std::string human_readable_string_;
 };
 
 // Drop stats for an xds cluster.