Kernel: Ignore unobserved BlockResult from Thread::Sleep

Suppress these in preparation for making BlockResult [[nodiscard]].

Kernel/Devices/USB/UHCIController.cpp

@@ -450,7 +450,7 @@ void UHCIController::spawn_port_proc()
                     }
                 }
             }
-            Thread::current()->sleep(sleep_time);
+            (void)Thread::current()->sleep(sleep_time);
         }
     });
 }

Kernel/Tasks/SyncTask.cpp

@@ -38,7 +38,7 @@ void SyncTask::spawn()
         dbgln("SyncTask is running");
         for (;;) {
             VFS::the().sync();
-            Thread::current()->sleep({ 1, 0 });
+            (void)Thread::current()->sleep({ 1, 0 });
         }
     });
 }