97eaadc3bf util: Remove `fsbridge::get_filesystem_error_message()` (Hennadii Stepanov)
Pull request description:
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](https://github.com/bitcoin/bitcoin/pull/20744) to `std::filesystem`, those discrepancies no longer exist.
2. It fails to display UTF-8 paths correctly on Windows:
```
> build\bin\Release\bitcoind.exe -datadir="C:\Users\hebasto\dd_₿_🏃" -regtest
...
2025-04-30T00:17:48Z DeleteAuthCookie: Unable to remove random auth cookie file: remove: Access is denied.: "C:\Users\hebasto\dd_?_??\regtest\.cookie"
...
```
3. It relies on `std::wstring_convert`, which was deprecated in C++17 and removed in C++26 (also see https://github.com/bitcoin/bitcoin/issues/32361).
This PR removes the obsolete `fsbridge::get_filesystem_error_message()` function, thereby resolving all of the above issues.
ACKs for top commit:
maflcko:
lgtm re-ACK 97eaadc3bf
davidgumberg:
untested crACK 97eaadc3bf
achow101:
ACK 97eaadc3bf
laanwj:
Code review ACK 97eaadc3bf
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68ac9f116c