util: Remove fsbridge::get_filesystem_error_message()

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.

src/util/fs.cpp

@@ -115,20 +115,4 @@ bool FileLock::TryLock()
 }
 #endif
 
-std::string get_filesystem_error_message(const fs::filesystem_error& e)
-{
-#ifndef WIN32
-    return e.what();
-#else
-    // Convert from Multi Byte to utf-16
-    std::string mb_string(e.what());
-    int size = MultiByteToWideChar(CP_ACP, 0, mb_string.data(), mb_string.size(), nullptr, 0);
-
-    std::wstring utf16_string(size, L'\0');
-    MultiByteToWideChar(CP_ACP, 0, mb_string.data(), mb_string.size(), &*utf16_string.begin(), size);
-    // Convert from utf-16 to utf-8
-    return std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>, wchar_t>().to_bytes(utf16_string);
-#endif
-}
-
 } // namespace fsbridge