[kernel] Remove last of old allocator interface

Removing the `allocator.h` file defining the `kutil::allocator`
interface, now that explicit allocators are not being passed around.
Also removed the unused `frame_allocator::raw_allocator` class and
`kutil::invalid_allocator` object.

Tags: memory

src/kernel/scheduler.h

@@ -3,7 +3,6 @@
 /// The task scheduler and related definitions
 
 #include <stdint.h>
-#include "kutil/allocator.h"
 #include "process.h"
 
 class lapic;