Delete the unused TestMemoryConfig target (#390)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/390

pybindings was the only target depending on this, but it didn't actually use it.

Noticed this while tracking down users of the non-span HierarchicalAllocator ctor.
ghstack-source-id: 201128783
exported-using-ghexport

Reviewed By: tarun292

Differential Revision: D49344929

fbshipit-source-id: 7435c75b554deb3a11e91d2ec9727ff6b82db7d3