[libc++] Remove <experimental/filesystem> header

`<filesystem>` header has been around for a while now, so we can safely remove
`<experimental/filesystem>` header. `_LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM`
suggests we were going to remove `<experimental/filesystem>` in llvm 11 release,
but we never did. So, remove the experimental header now, its associated tests,
and the `_LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM` macro.

Differential Revision: https://reviews.llvm.org/D119603