Kernel: Remove the kmalloc_eternal heap :^)

This was a premature optimization from the early days of SerenityOS.
The eternal heap was a simple bump pointer allocator over a static
byte array. My original idea was to avoid heap fragmentation and improve
data locality, but both ideas were rooted in cargo culting, not data.

We would reserve 4 MiB at boot and only ended up using ~256 KiB, wasting
the rest.

This patch replaces all kmalloc_eternal() usage by regular kmalloc().

Kernel/Memory/MemoryManager.h

@@ -137,7 +137,6 @@ private:
 };
 
 class MemoryManager {
-    AK_MAKE_ETERNAL
     friend class PageDirectory;
     friend class AnonymousVMObject;
     friend class Region;