Remove documentation on old arg in BackendDetails

Summary: Looks like the argument, "backend_debug_handle_generator," is no longer used and has been removed from the code, so remove the comment as well

Reviewed By: mergennachin, cccclai

Differential Revision: D54322749

fbshipit-source-id: 50574f8fa9f12ce14d577cd2105442a6f27670a2