relibc: remove unused imports in header/pthread/mod.rs

After fixing the Sys::open calls to use NulStr, the
redox_rt::proc::FdGuard and re-imported syscall were no
longer referenced. The cross-compile flagged them as
unused-import errors.

Per AGENTS.md zero-tolerance for warnings policy: clean
up the imports rather than suppress the warning.

src/header/pthread/mod.rs

@@ -5,12 +5,8 @@
 use alloc::collections::LinkedList;
 use core::{cell::Cell, mem::size_of, ptr::NonNull};
 
-#[cfg(target_os = "redox")]
-use redox_rt::proc::FdGuard;
 #[cfg(target_os = "linux")]
 use sc::syscall;
-#[cfg(target_os = "redox")]
-use syscall;
 
 use crate::{
     error::Errno,