chore(opentelemetry): remove some unused headers (#16500)

Author: scotthart

google/cloud/internal/call_context.h

@@ -18,11 +18,6 @@
 #include "google/cloud/internal/opentelemetry_context.h"
 #include "google/cloud/options.h"
 #include "google/cloud/version.h"
-#include <opentelemetry/trace/scope.h>
-#include <opentelemetry/trace/span.h>
-#include <opentelemetry/trace/tracer.h>
-#include <chrono>
-#include <functional>
 
 namespace google {
 namespace cloud {

google/cloud/internal/opentelemetry_context.h

@@ -18,8 +18,8 @@
 #include "google/cloud/version.h"
 #include <opentelemetry/context/context.h>
 #include <opentelemetry/context/runtime_context.h>
-#include <opentelemetry/trace/scope.h>
 #include <opentelemetry/trace/span.h>
+#include <utility>
 #include <vector>
 
 namespace google {