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/GlobalProcessExposed.cpp

@@ -403,7 +403,6 @@ private:
         JsonObjectSerializer<KBufferBuilder> json { builder };
         json.add("kmalloc_allocated", stats.bytes_allocated);
         json.add("kmalloc_available", stats.bytes_free);
-        json.add("kmalloc_eternal_allocated", stats.bytes_eternal);
         json.add("user_physical_allocated", system_memory.user_physical_pages_used);
         json.add("user_physical_available", system_memory.user_physical_pages - system_memory.user_physical_pages_used);
         json.add("user_physical_committed", system_memory.user_physical_pages_committed);