kernel/vm: vm_page::unused was really an unused field.

i.e. not an indicator of whether the page is unused. Rename it
as such and then remove the one usage of it (a KDL pretty-printer.)

headers/private/kernel/vm/vm_types.h

@@ -151,7 +151,7 @@ public:
 		// used in VMAnonymousCache::Merge()
 	bool					accessed : 1;
 	bool					modified : 1;
-	uint8					unused : 1;
+	uint8					_unused : 1;
 
 	uint8					usage_count;
 

src/system/kernel/vm/vm_page.cpp

@@ -780,7 +780,7 @@ list_page(vm_page* page)
 	if (page->busy_writing) kprintf("W"); else kprintf("-");
 	if (page->accessed)     kprintf("A"); else kprintf("-");
 	if (page->modified)     kprintf("M"); else kprintf("-");
-	if (page->unused)       kprintf("U"); else kprintf("-");
+	kprintf("-");
 
 	kprintf(" usage:%3u", page->usage_count);
 	kprintf(" wired:%5u", page->WiredCount());