Camera NDK: don't explicit delete ACameraMetadata

To fix the warning in RefBase destructor.

Test: NDK CTS tests, and ARCore measure app.
Bug: 112775599
Change-Id: Ia448ac6d7e6c17699c16d287f65fc2826c9d7578
diff --git a/camera/ndk/NdkCaptureRequest.cpp b/camera/ndk/NdkCaptureRequest.cpp
index ac1856b..ddb69d7 100644
--- a/camera/ndk/NdkCaptureRequest.cpp
+++ b/camera/ndk/NdkCaptureRequest.cpp
@@ -137,7 +137,7 @@
     if (request == nullptr) {
         return;
     }
-    delete request->settings;
+    request->settings.clear();
     delete request->targets;
     delete request;
     return;