test: remove redundant `using` in cctest

PR-URL: https://github.com/nodejs/node/pull/14739
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>

test/cctest/node_test_fixture.h

@@ -8,13 +8,6 @@
 #include "v8.h"
 #include "libplatform/libplatform.h"
 
-using node::Environment;
-using node::IsolateData;
-using node::CreateIsolateData;
-using node::CreateEnvironment;
-using node::AtExit;
-using node::RunAtExit;
-
 class ArrayBufferAllocator : public v8::ArrayBuffer::Allocator {
  public:
   virtual void* Allocate(size_t length) {