src: remove unused v8 namespace

PR-URL: https://github.com/nodejs/node/pull/32375
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_v8.cc

@@ -28,7 +28,6 @@ namespace node {
 
 using v8::Array;
 using v8::ArrayBuffer;
-using v8::BackingStore;
 using v8::Context;
 using v8::FunctionCallbackInfo;
 using v8::HeapCodeStatistics;