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/Graphics/FramebufferDevice.h

@@ -17,7 +17,6 @@
 namespace Kernel {
 
 class FramebufferDevice final : public GenericFramebufferDevice {
-    AK_MAKE_ETERNAL
     friend class DeviceManagement;
 
 public: