Kernel/FileSystem: Simplify the ProcFS significantly

Since the ProcFS doesn't hold many global objects within it, the need
for a fully-structured design of backing components and a registry like
with the SysFS is no longer true.

To acommodate this, let's remove all backing store and components of the
ProcFS, so now it resembles what we had in the early days of ProcFS in
the project - a mostly-static filesystem, with very small amount of
kmalloc allocations needed.
We still use the inode index mechanism to understand the role of each
inode, but this is done in a much "static"ier way than before.

Kernel/Thread.cpp

@@ -23,7 +23,6 @@
 #include <Kernel/Panic.h>
 #include <Kernel/PerformanceEventBuffer.h>
 #include <Kernel/Process.h>
-#include <Kernel/ProcessExposed.h>
 #include <Kernel/Scheduler.h>
 #include <Kernel/Sections.h>
 #include <Kernel/Thread.h>