[kernel] Get rid of page_manager

page_manager is dead - final uses replaced in vm_space (page_in and
clear). Removed the header and cpp, and other lingering references.

src/kernel/buffer_cache.cpp

@@ -2,7 +2,6 @@
 #include "buffer_cache.h"
 #include "kernel_memory.h"
 #include "objects/vm_area.h"
-#include "page_manager.h"
 #include "vm_space.h"
 
 using memory::frame_size;