The `fsbridge::get_filesystem_error_message()` function exhibits several
drawbacks:
1. It was introduced in https://github.com/bitcoin/bitcoin/pull/14192 to
account for platform-specific variations in
`boost::filesystem::filesystem_error::what()`. Since migrating to
`std::filesystem`, those discrepancies no longer exist.
2. It fails to display UTF-8 paths correctly on Windows.
3. It relies on `std::wstring_convert`, which was deprecated in C++17
and removed in C++26.
This change removes the `fsbridge::get_filesystem_error_message()`
function, thereby resolving all of the above issues.
Additionally, filesystem error messages now use the "Warning" log level.
GetErrorReason()'s Win32 implementation does the same thing as
Win32ErrorString(int err) from syserror.cpp, so call the latter.
Also remove now-unnecessary headers from sock.cpp and less verbose
handling of #ifdefs.
The fs.* files are already part of the libbitcoin_util library. With the
introduction of the fs_helpers.* it makes sense to move fs.* into the
util/ directory as well.
