access: file: simplify the check of remote path on Windows

PathIsNetworkPathW() is a basic function that only checks for \\ and [a-zA-Z]:\\ pathes [1]. It's also not available on UWP apps.

We can do the same by checking the UTF-8 string directly without even a conversion to wide char string.

[1] https://docs.microsoft.com/en-us/windows/win32/api/shlwapi/nf-shlwapi-pathisnetworkpathw