signal: remove await on thread.force_kill in x86 sigreturn

Prepare for x86 build fix.

Fixes: 59f044422cd4 ("mm, proc: add an exited thread reaper")
Signed-off-by: greatbridf <greatbridf@icloud.com>

src/kernel/syscall/procops.rs

@@ -876,7 +876,7 @@ async fn sigreturn() -> KResult<SyscallNoReturn> {
             "`sigreturn` failed in thread {} with error {err}!",
             thread.tid
         );
-        thread.force_kill(Signal::SIGSEGV).await;
+        thread.force_kill(Signal::SIGSEGV);
         return Err(err);
     }