[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/main.cpp

@@ -19,7 +19,6 @@
 #include "objects/channel.h"
 #include "objects/event.h"
 #include "objects/handle.h"
-#include "page_manager.h"
 #include "scheduler.h"
 #include "serial.h"
 #include "symbol_table.h"