fs: always try to get file type in fs::read_dir

[icedrocket]

The current implementation of fs::read_dir pre-stores the file type, except for a few operating systems that are known to require an extra call to get the file type. The cfg attributes we use to check such operating systems comes from the std implementation, but this was recently updated and no longer matches.

I think I'd rather just delete those cfg attributes, because when we read a directory, we also read the file type, except in very few cases.

[Darksonn]

but this was recently updated and no longer matches.

We must still support older compilers, so if these methods are not available there, then we cannot remove the cfgs yet.

[icedrocket]

but this was recently updated and no longer matches.

We must still support older compilers, so if these methods are not available there, then we cannot remove the cfgs yet.

The std implementation I linked to in my description is just the part that decides whether to make an extra call. Targets that support std should be fine, as they support DirEntry::file_type.

[icedrocket]

Superseded by #5806.