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/FileSystem/VirtualFileSystem.h

@@ -33,7 +33,6 @@ struct UidAndGid {
 };
 
 class VirtualFileSystem {
-    AK_MAKE_ETERNAL
 public:
     // Required to be at least 8 by POSIX
     // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html