Remove obsolete TODO comment (#374)

Author: psx95

propagators/gcp/src/main/java/com/google/cloud/opentelemetry/propagators/XCloudTraceContextPropagator.java

@@ -90,7 +90,6 @@ public <C> Context extract(Context context, C carrier, TextMapGetter<C> getter)
     if (Span.fromContext(context).getSpanContext().isRemote()) {
       return context;
     }
-    // TODO - Do we need to iterate over keys and lowercase them, or is java doing that for us?
     String value = getter.get(carrier, FIELD);
     if (value == null) {
       return context;