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().

Userland/Applications/SystemMonitor/MemoryStatsWidget.cpp

@@ -87,7 +87,6 @@ void MemoryStatsWidget::refresh()
     auto json_result = JsonValue::from_string(file_contents).release_value_but_fixme_should_propagate_errors();
     auto const& json = json_result.as_object();
 
-    [[maybe_unused]] u32 kmalloc_eternal_allocated = json.get("kmalloc_eternal_allocated").to_u32();
     u32 kmalloc_allocated = json.get("kmalloc_allocated").to_u32();
     u32 kmalloc_available = json.get("kmalloc_available").to_u32();
     u64 user_physical_allocated = json.get("user_physical_allocated").to_u64();