kernel/vm: Remove an obsolete comment.

page_num_t is typedef'd to phys_addr_t, so it's 64-bits on 32-bit
platforms with PAE. In fact it's been so since the introduction
of phys_addr_t, so this comment was obsolete from the start...

headers/private/kernel/vm/vm_types.h

@@ -123,8 +123,6 @@ private:
 public:
 	page_num_t				cache_offset;
 								// in page size units
-								// TODO: Only 32 bit on 32 bit platforms!
-								// Introduce a new 64 bit type page_off_t!
 
 	SplayTreeLink<vm_page>	cache_link;
 	vm_page*				cache_next;