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/Net/LoopbackAdapter.h

@@ -11,8 +11,6 @@
 namespace Kernel {
 
 class LoopbackAdapter final : public NetworkAdapter {
-    AK_MAKE_ETERNAL
-
 private:
     LoopbackAdapter(NonnullOwnPtr<KString>);