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/Heap/kmalloc.h

@@ -39,14 +39,12 @@ enum class align_val_t : size_t {};
 };
 
 void kmalloc_init();
-[[gnu::malloc, gnu::returns_nonnull, gnu::alloc_size(1)]] void* kmalloc_eternal(size_t);
 
 void kfree_sized(void*, size_t);
 
 struct kmalloc_stats {
     size_t bytes_allocated;
     size_t bytes_free;
-    size_t bytes_eternal;
     size_t kmalloc_call_count;
     size_t kfree_call_count;
 };