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/RamdiskDevice.h

@@ -16,7 +16,7 @@ class RamdiskController;
 class RamdiskDevice final : public StorageDevice {
     friend class RamdiskController;
     friend class DeviceManagement;
-    AK_MAKE_ETERNAL
+
 public:
     static NonnullRefPtr<RamdiskDevice> create(const RamdiskController&, NonnullOwnPtr<Memory::Region>&& region, int major, int minor);
     virtual ~RamdiskDevice() override;