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/Storage/StorageController.h

@@ -22,7 +22,6 @@ namespace Kernel {
 class AsyncBlockDeviceRequest;
 class StorageDevice;
 class StorageController : public RefCounted<StorageController> {
-    AK_MAKE_ETERNAL
 
 public:
     virtual ~StorageController() = default;