[kernel] Clean up includes in kernel logger

Drive-by include fixup as I was looking into how to teach the bootloader
to store its logs in the kernel's log memory.

src/kernel/logger.cpp

@@ -4,8 +4,6 @@
 
 #include "kassert.h"
 #include "logger.h"
-#include "objects/system.h"
-#include "objects/thread.h"
 
 // The logger is initialized _before_ global constructors are called,
 // so that we can start log output immediately. Keep its constructor