Kernel: Remove non existent friend class from Process.h

clang-tidy correctly flagged that ProcFSProcessOpenFileDescriptions
does not exist.

Kernel/Process.h

@@ -121,7 +121,6 @@ public:
 
     friend class Thread;
     friend class Coredump;
-    friend class ProcFSProcessOpenFileDescriptions;
 
     // Helper class to temporarily unprotect a process's protected data so you can write to it.
     class ProtectedDataMutationScope {