Remove utility cage from RawPOSIX

[rennergade]

There is a cage 0/utility cage that was used with RustPOSIX and its test suite that is no longer useful. Let's remove it from the codebase.

[Yaxuan-w]

FYI: https://github.com/Lind-Project/lind-wasm/blob/main/src/rawposix/src/sys_calls.rs#L1043