Get rid of SERENITY macro since the compiler already defines __serenity__

This makes it a bit easier to use AK templates out-of-tree.

AK/kmalloc.cpp

@@ -1,34 +1,11 @@
 #include "kmalloc.h"
 
-#ifndef SERENITY
+#ifndef __serenity__
 #include <cstdlib>
 #endif
 
 extern "C" {
 
-void* kcalloc(size_t nmemb, size_t size)
-{
-    return calloc(nmemb, size);
-}
 
-void* kmalloc(size_t size)
-{
-    return malloc(size);
-}
-
-void kfree(void* ptr)
-{
-    free(ptr);
-}
-
-void* krealloc(void* ptr, size_t size)
-{
-    return realloc(ptr, size);
-}
-
-void* kmalloc_eternal(size_t size)
-{
-    return kmalloc(size);
-}
 
 }