Remove duplicated PackedFunc C++ test (#8812)

I came across this file whilst looking at the C++ tests and realised it's a
duplicate of the PackedFunc tests which doesn't get invoked.

```
$ diff -u tests/cpp/contrib/bnns.cc tests/cpp/packed_func_test.cc
--- tests/cpp/contrib/bnns.cc   2021-07-30 12:59:33.830443830 +0000
+++ tests/cpp/packed_func_test.cc       2021-08-23 12:47:43.193708421 +0000
@@ -17,6 +17,13 @@
  * under the License.
  */

+#include <dmlc/logging.h>
+#include <gtest/gtest.h>
+#include <tvm/runtime/packed_func.h>
+#include <tvm/runtime/registry.h>
+#include <tvm/tir/expr.h>
+#include <tvm/tir/transform.h>
+
 TEST(PackedFunc, Basic) {
   using namespace tvm;
   using namespace tvm::tir;
```

Author: Mousius