Remove unused function from executorch/backends/apple/coreml/runtime/inmemoryfs/inmemory_filesystem.cpp (#4240)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4240

`-Wunused-function` has identified an unused function. This diff removes it. In many cases these functions have existed for years in an unused state.

Reviewed By: palmje

Differential Revision: D59644712

fbshipit-source-id: 7dc416d17ba8acf7251e8c60cf6e2abe2cf571a4